Photoinduced intramolecular charge transfer and S2 fluorescence in thiophene-π-conjugated donor-acceptor systems: Experimental and TDDFT studies

Article abstract of DOI:10.1002/chem.200701868

Experimental and theoretical methods were used to study newly synthesized thiophene-π-conjugated donor-acceptor compounds, which were found to exhibit efficient intramolecular charge-transfer emission in polar solvents with relatively large Stokes shifts and strong solvatochromism. To gain insight into the solvatochromic behavior of these compounds, the dependence of the spectra on solvent polarity was studied on the basis of Lippert-Mataga models. We found that intramolecular charge transfer in these donor-acceptor systems is significantly dependent on the electron-with-drawing at the thienyl 2-position. The dependence of the absorption and emission spectra of these compounds in methanol on the concentration of trifluoroacetic acid was used to confirm intramolecular charge-transfer emission. Moreover, the calculated absorption and emission energies, which are in accordance with the experimental values, suggested that fluorescence can be emitted from different geometric conformations. In addition, a novel S₂ fluorescence phenomenon for some of these compounds was also be observed. The fluorescence excitation spectra were used to confirm the S₂ fluorescence. We demonstrate that S₂ fluorescence can be explained by the calculated energy gap between the S₂ and Si states of these molecules. Furthermore, nonlinear optical behavior of the thiophene-π-conjugated compound with diethylcyanomethylphosphonate substituents was predicted in theory.

Full text of DOI:10.1002/chem.200701868

FULL PAPER
DOI: 10.1002/chem.200701868
Photoinduced Intramolecular Charge Transfer and S2 Fluorescence in
Thiophene-p-Conjugated Donor–Acceptor Systems: Experimental and
TDDFT Studies**
Guang-Jiu Zhao,[a] Rui-Kui Chen,[b] Meng-Tao Sun,[c] Jian-Yong Liu,[a] Guang-Yue Li,[a]
Yun-Ling Gao,[a] Ke-Li Han,*[a] Xi-Chuan Yang,*[b] and Licheng Sun*[b, d]
Abstract: Experimental and theoretical
methods were used to study newly syn-
drawing substituents at the thienyl 2-
position. The dependence of the ab-
sorption and emission spectra of these
compounds in methanol on the concen-
tration of trifluoroacetic acid was used
to confirm intramolecular charge-trans-
fer emission. Moreover, the calculated
absorption and emission energies,
which are in accordance with the ex-
perimental values, suggested that fluo-
rescence can be emitted from different
geometric conformations. In addition, a
novel S2 fluorescence phenomenon for
some of these compounds was also be
observed. The fluorescence excitation
spectra were used to confirm the S2
fluorescence. We demonstrate that S2
fluorescence can be explained by the
calculated energy gap between the S2
and S1 states of these molecules. Fur-
thermore, nonlinear optical behavior of
the thiophene-p-conjugated compound
with diethylcyanomethylphosphonate
substituents was predicted in theory.
thesized
thiophene-p-conjugated
donor–acceptor compounds, which
were found to exhibit efficient intramo-
lecular charge-transfer emission in
polar solvents with relatively large
Stokes shifts and strong solvatochrom-
ism. To gain insight into the solvato-
chromic behavior of these compounds,
the dependence of the spectra on sol-
vent polarity was studied on the basis
of Lippert–Mataga models. We found
that intramolecular charge transfer in
these donor–acceptor systems is signifi-
cantly dependent on the electron-with-
Keywords: charge transfer · density
functional calculations · donor–ac-
ceptor systems
nonlinear optics
· fluorescence ·
Introduction
nitrile (DMABN), one of the donor–acceptor p-conjugated
(D-p-A) compounds, has led to numerous theoretical and
experimental studies to explore the origin of intramolecular
charge-transfer (ICT) fluorescence,[2–4] which plays a key
Since the first observation by Lippert et al.,[1] the phenom-
enon of dual fluorescence for 4-(N,N-dimethylamino)benzo-
[a] G.-J. Zhao,+ Dr. J.-T. Liu, G.-Y. Li, Dr. Y.-L. Gao, Prof. Dr. K.-L. Han
State Key Laboratory of Molecular Reaction Dynamics
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
[c] Dr. M.-T. Sun
National Laboratory for Condensed State Matter Physics
Institute of Physics, Chinese Academy of Sciences
P. O. Box 603-146, Beijing 100080 (China)
457 Zhongshan Road, Dalian 116023 (China)
Fax : (+86)411-84675584
E-mail: gjzhao@dicp.ac.cn
[d] Prof. Dr. L. Sun
KTH Chemistry, Organic Chemistry
Royal Institute of Technology
10044 Stockholm (Sweden)
Fax : (+46)8-7908127
klhan@dicp.ac.cn
[b] R.-K. Chen,+ Dr. X.-C. Yang, Prof. Dr. L. Sun
State Key Laboratory of Fine Chemicals
Dalian University of Technology
158 Zhongshan Road, Dalian 116012 (China)
Fax : (+86)411-83702185
[+] These authors contributed equally to this work.
[**] TDDFT=Time-dependent density functional theory.
E-mail: yangxc@dlut.edu.cn
lichengs@kth.se
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role in the photophysics of D-p-A compounds. After the
finding of the importance of photoinduced ICT processes in
biological systems such as photosynthesis,[5] interest in mo-
lecular donor–acceptor systems has increased significant-
ly.[6–14] For several years, the dual-fluorescence properties of
donor–acceptor systems have been used for studying molec-
ular switches.[15–19] It was found that changes in molecular
structure and conjugated system can induce very different
optical and physical properties in D-p-A compounds.[20–51]
We have reported on the design, synthesis, and photo-
physical properties of a series for thiophene-p-conjugated
D-p-A compounds culminating in 1-cyano-2-{5-[2-(1,2,2,4-
tetramethyl-1,2,3,4-tetrahydroquinolin-6-yl)vinyl]thiophen-2-
yl}vinylphosphonic acid diethyl ester (QTCP, Scheme 1).[52]
In electronic spectral studies, QTCP was found to exhibit ef-
ficient ICT emission in polar solvents with large Stokes shift
and strong solvatochromism.[52] Two main models have been
proposed to explain the mechanism of formation of the in-
tramolecular charge-transfer state: twisted intramolecular
charge transfer (TICT) and planar intramolecular charge
transfer (PICT).[4] In our previous investigations, detailed
evidence on the ICT state revealed that, in the electronically
excited state, charge transfer from the donor moiety
(TMTHQ) to the electron-withdrawing species is accompa-
nied by an anomalous 908 twist of the donor compound
(TMTHQ) relative to the thiophene p bridge.[52] Hence, our
results confirm the TICT model. Furthermore, these studies
have revealed that the thiophene bridge is an ideal building
block for construction of D-p-A compounds with ICT prop-
erties.[53]
cells (DSSCs).[54–59] Since thiophene has a lower delocaliza-
tion energy than benzene, it can offer better effective conju-
gation than benzene in donor–acceptor compounds.[59] To
get a better understanding of the influence of different sub-
stituents on intramolecular charge transfer (ICT), we have
now synthesized and fully characterized several derivatives
of 1,2,2,4-tetramethyl-1,2,3,4-tetrahydroquinoline (TMTHQ)
with different electron acceptors and thiophene p bridges.
For comparison, benzene-bridged analogue QBCP (see
Scheme 1) was also studied.
We first investigated the TMTHQ systems using steady-
state absorption and fluorescence spectroscopy. Differences
in absorption and fluorescence spectra for QT, QTC, QTCP,
and QBCP in various solvents are discussed in detail. The
fluorescence excitation spectra were also recorded to con-
firm the novel S2 fluorescence phenomenon. Subsequently,
the influence of adding of trifluoroacetic acid (TFA) to solu-
tions of TMTHQs on their absorption and emission spectra
were investigated. For better interpretation of our experi-
mental results, the excited states of all the TMTHQ com-
pounds considered were studied by time-dependent density
functional theory (TDDFT).
Results and Discussion
Absorption spectra: Typical UV/Vis absorption spectra of
QT, QTC, QTCP, and QBCP in different solvents at room
temperature are shown in Figure 1. The strong absorption
maximum in the visible region and relatively weak absorp-
!
It is well known that different substituents in electron-
donor and -acceptor moieties will affect the electron distri-
bution in the whole molecule and thus result in different
photophysical and photochemical properties.[20,25–27] The p-
conjugated bridge that connects the electron donor with the
acceptor also influences the integrated characteristics.[25,27]
Compounds containing a thiophene ring have been widely
investigated with the aim of developing various optoelec-
tronic devices such as organic light-emitting diodes
(OLEDs), nonlinear optics (NLO), and dye-sensitized solar
tion peak in the near UV region correspond to the S1 S0
!
and S2 S0 electronic transitions, respectively. Moreover, all
compounds in different solvents display intense and wide
absorption bands. In general, an electron-withdrawing sub-
stituent at the thienyl 2-position can induce a remarkable
red shift of the absorption maximum, the origin of which
can be attributed to the ICT state for a strong push–pull
system.[4–7] The broad absorption band can be assigned to
electronic transitions delocalized throughout the whole mol-
ecule.[4] The thienylethyl and benzene p-bridging units en-
Scheme 1. a) tBuOK, diethyl 2-thienylmethylphosphonate, THF, 08C, 2 h, 95%; b) nBuLi, DMF, THF, À158C, 56%; c) diethyl cyanomethylphosphonate,
CH3CN, piperidine, reflux, 2 h; d) tBuOK, diethyl benzylphosphonate, THF, 08C, 2 h, 64%; e) DMF, POCl3, CH2Cl2, RT, 24 h, 53%.
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Thiophene-p-Conjugated Donor–Acceptor Systems
FULL PAPER
Fluorescence spectra: The fluorescence spectra of all com-
pounds exhibit fine structure in hexane but become struc-
tureless in diethyl ether and in more polar solvents (see
Figure 2). In addition, unlike the absorption spectra, the
fluorescence spectra of QTC and QTCP exhibit significant
redshifts in polar solvents, indicative of a pronounced ICT
characteristic of the fluorescent states.[52] As in the case of
QT, the maxima of the emission spectra extend from 402 nm
in hexane to 464 nm in MeCN, that is, a comparatively small
solvatochromic shift of 62 nm. With an electron-withdrawing
formyl substituent in the thienyl 2-position, the emission
maxima of QTC range widely from 474 nm (in hexane) to
666 nm (in methanol), and the solvatochromic shift of
QTCP becomes even as large as 224 nm on changing from
hexane to methanol. This indicates that the magnitude of
the solvatochromic shift also depends on the electron-with-
drawing substituent at the thienyl 2-position in thiophene-p-
conjugated TMTHQs. The smaller solvatochromic shift of
QBCP with QTCP confirms that a thiophene bridge can
offer more effective conjugation than a benzene bridge in
donor–acceptor compounds.[59] Figure 2 shows that solvent
polarity plays an important role in the fluorescence of QTC
and QTCP in solution. In the fluorescent state, polar sol-
vents can induce ICT, which is followed by geometric twist-
ing.[52] Thus, fluorescence is emitted from the TICT state in
polar solvents.[4,52] In addition, the fluorescence of QTC and
QTCP in nonpolar solvents is emitted from the fluorescent
states with planar conformations.[52]
S2 fluorescence: Detailed absorption and fluorescence spec-
tra of QTCP at different excitation wavelengths (Figure 3)
clearly show the fine structure of the electronic transitions.
Figure 1. UV/Vis absorption spectra of QT (a, b), QTC (c, d), QBCP (e,
f), and QTCP (g, h) in hexane (solid curves) and in acetonitrile (dashed
curves).
large the conjugated system, so
the absorption bands of QTC,
QTCP, and QBCP are slightly
broader than that of QT. In ad-
dition, the solvatochromic shift
is rather small for QT (365 nm
in hexane and 370 nm in aceto-
nitrile). This indicates that the
difference in dipole moments of
the Franck–Condon (FC) excit-
ed state and the ground state of
QT is quite small.[5] However,
the solvatochromic shifts of
QTC, QTCP, and QBCP are
larger due to the larger differ-
ence in dipole moment of the
FC excited state and the
ground state, which is induced
by the electron-withdrawing
formyl and diethylcyanomethyl-
phosphonate substituents.
Figure 2. Normalized fluorescence spectra in a) hexane, b) diethyl ether, c) dichloromethane, d) acetone,
e) DMF, f) acetonitrile, and g) methanol.
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6937

K.-L. Han, X.-C. Yang, L. C. Sun et al.
the S2 and S1 fluorescence states is significantly larger in
polar solvents than in nonpolar solvents.
To confirm S2 fluorescence, fluorescence excitation spec-
tra for the two fluorescence peaks of QTCP were recorded
(Figure 4). The fluorescence excitation spectra for the long-
Figure 3. Normalized UV/Vis absorption (dashed lines) and emission
spectra (solid lines) of QTCP in hexane (black) and methanol (gray).
The emission spectra were recorded after excitation at 330 nm (A) and at
the longest absorbance wavelength (B).
In addition to the strong absorption maxima in the visible
region, a relatively weak absorption peak in the UV region
can also be seen. The strong absorption peak corresponds to
Figure 4. Fluorescence excitation spectra for S1 fluorescence (black) and
S2 fluorescence (gray) of QTCP in hexane (A) and methanol (B).
!
the S1 S0 electronic transition, and the relatively weak
!
peak to the S2 S0 electronic transition. The two absorption
wavelength fluorescence of QTCP in both hexane and meth-
bands, which are well resolved, may be due to the large
energy gap between the S2 and S1 states.[31–38,59] Moreover,
the weak peak has a smaller solvatochromic shift than the
absorption maximum in more polar solvents. The weak peak
is located at 336 nm in hexane and at 345 nm in methanol,
while the absorption maximum is at 473 and 510 nm in
hexane and methanol, respectively. Thus, compared with
QTCP in hexane, the energy gap between the S2 and S1
states is larger in methanol because of better stabilization of
the S1 state in polar solvents.[4] This suggests that the S1 state
is more polar than the S2 state due to the ICT character of
the S1 state.
anol are similar to the corresponding absorption spectra, in
!
which the S1 S0 electronic transition band is much stronger
!
!
than the S2 S0 transition band. However, only the S2 S0
electronic transition band can be observed in the fluores-
cence excitation spectra for the short-wavelength fluores-
!
cence, since the energy of S1 S0 electronic transition is
lower than that of the short-wavelength fluorescence. The
short-wavelength fluorescence can only originate from the
S2 state. Thus, the S2 fluorescence of QTCP is confirmed by
the fluorescence excitation spectra.
Figure 5 shows the fluorescence emission spectra of QT
and QTC for excitation at the wavelength corresponding to
!
The fluorescence spectra for excitation at different wave-
the S2 S0 transition. Only the S1 fluorescence can be ob-
!
!
lengths corresponding to the S1 S0 and S2 S0 electronic
transitions, respectively, are also shown in Figure 3. A novel
feature was found in the fluorescence spectrum of QTCP
after photoexcitation at 336 nm. In addition to the S1!S0
emission band, we also observed the appearance of a new
served for QT and QTC in both hexane and methanol. This
may be ascribed to the small energy gap between the S2 and
S1 states, which facilitates nonradiative transition from the
S2 state to the S1 state. Thus, S2 fluorescence of QT and
QTC molecules is not found.
The fluorescence emission and excitation spectra of
QBCP in hexane and methanol are shown in Figure 6. In
the fluorescence emission spectra with excitation at the S1
absorption wavelength, only the S1 fluorescence peak is ob-
served in both hexane and methanol. When QBCP in
hexane is excited at the S2 absorption wavelength, both S1
!
emission band of low intensity on the blue side of the S1 S0
absorption band. Thus, we relate the origin of the new emis-
sion band to the S2!S0 fluorescence.[37,59] The Stokes shift
of the S1 state in polar solvents is larger than that of the S2
state, and this could indicate ICT character for the S1 state
and LE nature for the S2 state.[4] The energy gap between
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Thiophene-p-Conjugated Donor–Acceptor Systems
FULL PAPER
relatively strong S2 fluorescence peak can be found in the
fluorescence emission spectra of QBCP in methanol. It can
be suggested that the energy gap between S2 and S1 states of
QBCP is larger than that of QTCP. Furthermore, the S2
fluorescence of QBCP is also confirmed by the fluorescence
excitation spectra shown in Figure 6B.
Lippert–Mataga analysis: The photophysical properties of
all compounds in various solvents are listed in Table 1. The
Stokes shifts of all compounds are strongly dependent on
both the electron-withdrawing substituents at the thienyl 2-
position and the p-bridge substituents. The Stokes shifts of
QTC and QTCP in polar solvents are significantly larger
than those of QT and QBCP in the corresponding solvents.
Moreover, the Stokes shifts of QTC and QTCP increase
drastically with increasing the polarity of the solvents, while
those of QT and QBCP depend only slightly on the solvent.
The large Stokes shift can be ascribed to the ICT character-
istics of the fluorescent states of QTC and QTCP. The mag-
nitudes of the shifts from hexane to methanol are in the
order of QTCP>QTC >QBCP>QT, which roughly paral-
lels the relative electron-withdrawing ability of the substitu-
ents.[4,20] By using Ff and tf values, radiative (kr =Ff/tf) and
nonradiative (knr =(1ÀFf)/tf) rates were calculated. The Ff
Figure 5. Fluorescence emission spectra with excitation at the wavelength
corresponding to the S2 absorption band. A) QT: 270 nm excitation;
B) QTC: 310 nm excitation.
Table 1. Absorption (lab [nm]) and emission (lem [nm]) maxima, Stokes
shifts (Dlst [nm]), fluorescence quantum yield (Ff) and lifetimes (tf [ns]),
radiative (kr [107 sÀ1]), and nonradiative (knr [107 sÀ1]) rates for all com-
pounds in different solvents.
Solv
lab
lem
Dlst
Ff
tf
kr
knr
hexane
Et2O
CH2Cl2
acetone
DMF
366
368
375
372
376
371
368
402
423
454
454
462
464
457
36
55
79
82
86
93
89
0.211
0.171
0.126
0.137
0.032
0.104
0.090
3.7
3.9
5.0
4.3
5.1
4.4
4.8
5.70
4.38
2.52
3.19
0.63
2.36
1.88
21.3
21.3
17.5
20.1
19.0
20.4
19.0
QT
MeCN
MeOH
hexane
Et2O
CH2Cl2
acetone
DMF
473
486
509
500
511
500
510
544
623
687
697
718
721
734
71
0.144
0.249
0.181
0.099
0.048
0.050
0.022
3.2
3.6
6.3
5.3
4.3
3.9
2.8
4.50
6.92
2.87
1.87
1.12
1.28
0.79
26.8
20.9
13.0
17.0
22.1
24.4
34.9
137
178
197
207
221
224
QTCP
MeCN
MeOH
Solv
lab
lem
Dlst
Ff
t2
kr
knr
hexane
Et2O
CH2Cl2
acetone
DMF
425
432
453
442
449
444
448
474
533
601
603
618
627
666
49
0.066
0.069
0.143
0.160
0.142
0.146
0.015
3.4
3.2
3.2
3.5
3.4
3.5
2.8
1.94
2.16
4.47
4.57
4.18
4.17
0.54
27.5
29.1
26.8
24.0
25.2
24.4
35.2
101
148
161
169
183
218
QTC
MeCN
MeOH
Figure 6. A) Fluorescence emission spectra with excitation at the wave-
length corresponding to the S1 and S2 absorption bands for QBCP in
hexane (dashed lines) and in methanol (solid lines). B) Fluorescence ex-
citation spectra for S1 fluorescence (black) and S2 fluorescence (gray) of
QBCP in hexane (dash lines) and methanol (solid lines).
hexane
Et2O
CH2Cl2
acetone
DMF
429
439
462
454
463
457
471
505
519
552
553
564
563
565
76
80
90
99
101
106
94
0.224
0.276
0.064
0.012
0.008
0.007
0.001
5.4
3.3
3.1
3.3
3.3
3.0
2.9
4.15
8.36
2.06
0.36
0.24
0.23
0.03
14.4
21.9
30.2
29.9
30.1
33.1
34.5
QBCP
MeCN
MeOH
and S2 fluorescence is observed. Moreover, S2 fluorescence
is increased and comparable to S1 fluorescence. Only the
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K.-L. Han, X.-C. Yang, L. C. Sun et al.
fðeÞ ¼ ðeÀ1Þ=ð2e þ 1Þ
fðn2Þ ¼ ðn2À1Þ=ð2n2 þ 1Þ
ð2Þ
ð3Þ
ð4Þ
and kr values show that the compounds are less fluorescent
in more polar solvents, especially in alcoholic solvents.
Moreover, the nonradiative rate is correspondingly in-
creased. Thus, nonradiative ICT process of the compounds
can be facilitated by more polar solvents, especially protic
alcoholic solvents, which can provide intermolecular hydro-
gen-bonding interactions.
The progressive increase of the Stokes shift with increas-
ing solvent polarity can also be regarded as an indication of
the increase in dipole moment from the ground state to the
excited state.[20] To gain better insight into the solvatochro-
mic behavior of all compounds, the spectral dependence on
solvent polarity was studied on the basis of the Lippert–
Mataga models,[68,69] (Figure 7). The dipole moment me of
1=3
a ¼ ð3m=4NpdÞ
where e is the dielectric constant, n the refractive index, and
a the Onsager radius of the solute, which can be derived
from the Avogadro number N, molecular weight M, and
density d. For an LE state, the applicable polarity function f-
G
G
an ICT state. The dipole moments of the fluorescent state
obtained by fitting Equation (1) and the values of some
other parameters are listed in Table 2. The calculated dipole
moments of the ground and fluorescence states are both in
the order QTCP>QTC>QBCP>QT.
Table 2. Dipole moments of ground state and electronically excited
states for QT, QTC, QTCP, and QBCP.
[b]
a [Š][a]
mf [cmÀ1
]
mg [D][c]
me [D]
QT
4.90
5.05
5.77
5.74
À10856.2
À18397.7
À15453.4
À7188.8
3.99
10.14
11.23
10.03
8.32
21.2
23.7
17.2
QTC
QTCP
QBCP
[a] Onsager radius calculated by Equation (4) with d=1.0 gcmÀ3 for QT,
QTC, QTCP, and QBCP. [b]Calculated on the basis of Equation (1).
[c] Calculated at the B3LYP/TZVP level.
Effect of added trifluoroacetic acid: The dependence of the
absorption and emission spectra of thiophene-p-conjugated
QT, QTC, and QTCP in methanol on the concentration of
trifluoroacetic acid (TFA) was investigated (Figure 8). For
all compounds, addition of TFA leads to disappearance of
the original absorption bands and formation of a new band
at shorter wavelength. The new absorption bands can be at-
tributed to absorption of protonated forms of the com-
pounds. Moreover, protonation at the amino group can
eliminate the ICT process in these compounds, and the fluo-
rescence process can then only occur from the protonated
forms with planar structures.[70,71] Thus, fluorescence emis-
sion from the TICT fluorescence state of QTC and QTCP
in polar solvents can disappear after they are completely
protonated. The emission spectra were recorded after exci-
tation around the maximum of the absorption spectra in the
absence of TFA and excitation around the newly formed ab-
sorption band when TFA was added.
Generally, the emission spectra in the presence of TFA
should have two bands corresponding to the protonated and
nonprotonated forms.[70] In the case of QT, the emission
spectra in the presence of TFA show only one emission
band, whereby the two bands from the protonated and non-
protonated forms are not resolved. This confirms that the
fluorescent state for QT is of LE character. However, there
are two unambiguous bands for QTC and QTCP in metha-
nol in the presence of TFA. The added TFA induces de-
creased intensity of the original emission bands and forma-
tion of new emission bands at higher frequency, which can
Figure 7. Correlation diagram of the energies of the fluorescence maxima
against the Lippert–Mataga solvent parameter f(e,n).
A
the fluorescent state can be estimated from the slope mf of
the plot of the energy of the fluorescence maxima nf against
the solvent parameter f(e,n) [Eqs. (1)–(4)].
E
vf ¼ À½ð1=4pe0Þð2hca3ފ½meðmeÀmgފfðe,nÞ þ Const:
ð1Þ
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Thiophene-p-Conjugated Donor–Acceptor Systems
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ter than QTC. In the presence of TFA, both QTC and
QTCP in methanol exhibit decreases in ICT emission and
new emissions from the protonated form. In addition, the
newly formed emission bands for the protonated forms are
significantly shifted with respect to the emission bands of
the nonprotonated forms. This confirms the TICT nature of
the original fluorescent states for QTC and QTCP in metha-
nol before adding TFA. For QT in solution, however, the
emission more likely originates from the LE state.
Optimized structures: Structures of ground and electronical-
ly excited states of all compounds in planar and twisted con-
formations were optimized. Some important bond lengths,
bond angles, and dihedral angles in the optimized structures
are listed in Table 3. The benzene and thiophene rings are
nearly in a plane in all ground-state structures. The dihedral
angles in the ground states reveal that electron-withdrawing
groups make the molecules more planar. In addition,
bonds 2 and 4 are slightly shortened after adding electron-
withdrawing groups to QT, while bond 3 is lengthened. The
most significant feature is the structural changes in electron-
ically excited states compared to the ground state. In partic-
ular, bond 2 is slightly shortened in both S1 and S2 states for
QT and QBCP. This bond is slightly lengthened in the S1
state but slightly shortened in the S2 state for QTC and
QTCP. Bond 2 is significantly lengthened in the TICT states
of QTC and QTCP. The molecular backbones of QTC and
QTCP are less planar in the S1 state than in the ground
state according to the calculated dihedral angles. On the
contrary, the backbones of QTC and QTCP are more planar
in the S2 state than in the ground state. Moreover, the struc-
tures of QT and QBCP in both S1 and S2 states are always
more planar than in the ground state. Therefore, the less
planar molecular backbones of QTC and QTCP in the S1
state may indicate that ICT takes place in this state.
Figure 8. Effect of protonation on the absorption and emission spectra of
QT, QTC, and QTCP in methanol at 297 K.
be assigned to the protonated form.[71] Since protonation at
the amino group can lead to elimination of the charge-trans-
fer process in the molecule,[70,71] the size of the emission
shift from the nonprotonated form to the protonated form
can be used to measure the ICT property of the fluorescent
state. The shift of emission spectra for QTC in methanol
due to protonation is as large as about 200 nm. However,
the shift for QTCP in methanol is even larger (about
300 nm). Thus, QTCP in methanol shows better ICT charac-
Molecular orbitals: Frontier molecular orbitals (MOs) are
displayed in Figure 9 for the planar (all compounds) and
twisted (only QTC and QTCP) conformations. According to
Table 3. Optimized structural parameters for QT, QTC, QTCP, and QBCP in different electronic states in planar and twisted conformations. L: bond
lengths [Š]; A: bond angles [8]; DA: dihedral angles [8].
QT
QTC
QTCP
planar
QBCP
planar
S1
planar
S1
twisted
TICT
twisted
TICT
planar
S1
GS
S2
GS
S2
GS
S1
S2
GS
S2
L(1)
L(2)
L(3)
L(4)
1.406
1.456
1.348
1.447
1.377
119.2
127.5
125.5
131.3
3.612
2.629
1.422
1.425
1.398
1.404
1.410
120.1
124.5
124.8
129.5
2.336
1.286
1.423
1.424
1.378
1.421
1.394
117.6
127.7
125.6
131.7
4.788
2.359
1.407
1.451
1.351
1.442
1.388
119.2
127.7
125.0
130.6
1.890
0.331
1.412
1.452
1.369
1.426
1.397
119.5
124.9
126.9
129.9
8.027
3.670
1.412
1.442
1.369
1.412
1.413
119.4
127.2
125.8
130.0
1.196
0.299
1.415
1.499
1.374
1.435
1.416
121.2
122.8
128.8
130.3
105.3
179.6
1.408
1.449
1.353
1.439
1.390
119.2
127.7
124.9
130.5
1.178
0.552
1.410
1.454
1.360
1.436
1.386
119.4
125.5
126.7
130.4
9.777
2.972
1.415
1.436
1.379
1.417
1.413
119.5
126.4
124.2
129.6
1.091
0.572
1.406
1.485
1.353
1.428
1.393
121.3
122.7
128.3
130.0
105.2
179.8
1.407
1.452
1.350
1.454
1.411
119.2
127.7
126.7
123.9
1.222
0.306
1.408
1.462
1.350
1.464
1.406
118.9
126.1
127.5
123.2
0.978
0.588
1.415
1.438
1.378
1.435
1.422
119.2
127.0
125.8
124.3
1.259
0.538
L(5)
A(12)
A(23)
A(34)
A(45)
DA
DA
ACHTREUNG
ACHTREUNG
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6941

K.-L. Han, X.-C. Yang, L. C. Sun et al.
Figure 9. Frontier MOs for different species in planar and twisted conformations. H: HOMO; L: LUMO.
our TDDFT calculations, the S1 state corresponds to the
HOMO!LUMO transition, and the S2 state to the
HOMOÀ1!LUMO transition. Thus, only the HOMOÀ1,
HOMO, and LUMO orbitals in the planar and twisted struc-
tures are shown here. For all the molecules in planar confor-
mation the electron densities of all the orbitals shown here
are delocalized over the whole molecule. This can contribute
to broad absorption bands in the absorption spectra of all
compounds. For the perpendicular geometric structures, the
electron density of the HOMO is localized on the donor
moiety, while the electron densities of both LUMO and
HOMOÀ1 orbitals are only localized on the acceptor
moiety. Thus, the ICT nature of the S1 state and the LE
character of S2 state are demonstrated.
Density of states: The calculated total density of states
(TDOS) and the projected density of states (PDOS) for
QTCP in planar and twisted conformations are shown in
Figure 10. The PDOS can provide insight into the roles of
different fragments in the QTCP molecule.[72,73] The most
significant feature of the DOS is that HOMOÀ1 has similar
contributions of fragments to the LUMO, while the HOMO
is very different from the LUMO. The main contributions to
the HOMOÀ1 and LUMO come from the thiophene ring
and cyano group, while the contributions from the benzene
ring and the nitrogen atom of the donor are relatively small,
especially in the twisted conformation. This suggests that
the transition from HOMOÀ1 to LUMO is not followed by
a drastic intramolecular charge redistribution between dif-
ferent fragments.[4] Thus, the S2 state, which corresponds to
Figure 10. Total density of states (TDOS) and projected density of states
(PDOS) for a) planar QTCP and b) twisted QTCP.
the HOMOÀ1!LUMO transition, is an LE state. However,
the benzene ring and nitrogen atom of the donor make the
main contributions to the HOMO. Clearly, the transition
from the HOMO to the LUMO can be followed by charge
decrease on the benzene ring and nitrogen atom of the
donor and charge increase on the thiophene ring and cyano
group, so the ICT nature of the S1 state is evident.
sorption and fluorescence electronic excitation energies for
all compounds in planar conformation and twisted QTC and
QTCP are presented. All the calculated excitation energies
coincide well with the experimental values. Comparison of
the calculated and experimental S1 fluorescence emission
energies shows that the S1 fluorescence of QTC and QTCP
in nonpolar solvents is emitted from the excited states in the
Calculated excitation energies and energy gap between S2
and S1 states: In Table 4, calculated and experimental ab-
6942
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Chem. Eur. J. 2008, 14, 6935 – 6947

Thiophene-p-Conjugated Donor–Acceptor Systems
FULL PAPER
Table 4. Calculated absorption and fluorescence electronic excitation energies [nmÀ1] for all compounds and
corresponding experimental values (in parentheses) in planar (in hexane) and twisted (in acetonitrile) confor-
mations.
tures. However, polar solvents
can decrease the energy barrier,
and thus photoexcited QTCP
can pass through the energy
barrier easily.[44,45] Hence, S1
fluorescence is preferably emit-
ted from the perpendicular geo-
metric structure for QTCP in
polar solvents. According to the
QT
QTC
QTCP
QBCP
planar
planar
twisted
planar
twisted
planar
S1
S2
381 (366)
309 (268)
0.764
425 (402)
–
442 (425)
352 (303)
0.722
462 (474)
–
–
–
–
497 (473)
363 (336)
0.918
521 (544)
389 (383)
–
–
–
491 (429)
343 (266)
1.081
Abs.
Flu.
DE[a]
S1
587 (627)
–
701 (721)
–
518 (505)
S2
361(367)
A
potential-energy
curve
of
[a] DE is the energy gap [eV] between the S1 and S2 states.
planar conformation, while in polar solvents (e.g. in acetoni-
trile) it is emitted from the ICT state in the twisted confor-
mation.[4,52] Thus, the polarity of solvents plays an important
role for S1 fluorescence of QTC and QTCP in solution.[4] In
the S1 fluorescent state, the solvent polarity can induce ICT
followed by geometric twisting,[52] while the fluorescence of
QT and QBCP is emitted from the planar geometric struc-
tures, since no evident ICT process can occur. In addition,
site-specific intermolecular hydrogen bonding interactions
can also influence the electronic spectra, such as quenching
fluorescence.[60–67]
As discussed above, the S2 fluorescence can be observed
for QTCP and QBCP but not for QT and QTC. This may
be correlated with the energy gap between S2 and S1
states.[59] Thus, the calculated energy gaps between S2 and S1
states for all the compounds are also listed in Table 4. The
calculated energy gap between S2 and S1 states of QTCP is
as high as 0.918 eV. The large energy gap between S2 and S1
states can decrease the rate of the internal conversion from
S2 state to S1 state as well as some other nonradiative pro-
cesses.[59,63] So the S2 state of QTCP molecule can emit fluo-
rescence. The energy gaps of QT and QTC are very close
and much smaller than that of QTCP. The small energy gap
between S2 and S1 states increases the electronic coupling of
the two electronically excited states, and thus facilitates in-
ternal conversion from the S2 state to S1 state.[59,63] Hence,
the S2 fluorescence for QT and QTC cannot be measured.
The energy gap between the S2 and S1 states of QBCP is
larger than that of QTCP; thus, QBCP has strong S2 fluo-
rescence.
Figure 11. Calculated potential-energy curves as a function of twisting
angle for different electronic states for all compounds.
QTCP, the fluorescence of the TICT state is drastically red-
shifted, which is consistent with the experimental value. In
contrast, the S2 the twisting-angle potential-energy curve has
a maximum at the perpendicular geometry, so S2 fluores-
cence is preferably emitted from the planar geometry. The
potential-energy curves of QTC are also of TICT character,
while the energy gap between S2 and S1 states is smaller
than that of QTCP. Thus, the S2 fluorescence emission
cannot be observed. However, the potential-energy curves
of QT and QBCP differ from that of QTCP and QTC. Both
QT and QBCP with perpendicular geometries are unstable
in all electronic states. Hence, fluorescence of QT and
QBCP can only be emitted from the planar conformations.
Table 5
Potential-energy curves: Potential-energy curves as a func-
tion of the twisting angle were calculated for the ground
and low-lying excited states of all compounds (Figure 11).
The potential-energy curves of QTCP are representative of
TICT character.[44,45] In the ground state, QTCP prefers a
planar structure, and the twisted structure is unstable. On
photoexcitation, planar QTCP can be initially excited to the
S1 or S2 state of planar geometry. The potential-energy
curve of the S1 state has a minimum at the perpendicular ge-
ometry. Generally, there is an energy barrier between the
planar and perpendicular structures in the S1 state.[44,45] Pho-
toexcited QTCP in nonpolar solvents cannot pass through
the energy barrier in the S1 twisting-angle potential-energy
curve and only emits fluorescence from the planar struc-
Effects of protonation: The QT, QTC, and QTCP molecules
protonated at the nitrogen atom of the donor moiety were
also investigated here and the results compared with experi-
mental data. Protonation at the amino group can eliminate
the ICT process of the compounds, and then fluorescence
can only occur from the protonated forms with planar struc-
ture. Thus, only the planar conformations for the protonated
QT, QTC, and QTCP molecules are presented here. The
calculated electronic excitation energies and the oscillator
Chem. Eur. J. 2008, 14, 6935 – 6947
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www.chemeurj.org
6943

K.-L. Han, X.-C. Yang, L. C. Sun et al.
Table 5. Calculated absorption electronic excitation energies [nm] and
corresponding oscillator strengths (in parentheses) for protonated com-
pounds in the planar conformation.
Protonated
Protonated
Protonated
QT
QTC
QTCP
S1
S2
S3
S4
S5
373 (0.966)
349 (0.065)
307 (0.015)
285 (0.009)
263 (0.102)
396 (0.000)
380 (1.209)
336 (0.030)
311 (0.015)
288 (0.082)
429 (1.520)
359 (0.020)
329 (0.127)
317 (0.006)
313 (0.053)
strengths for corresponding electronic states are listed in
Table 5. The absorption maxima are located at 373, 380, and
429 nm for protonated QT, QTC, and QTCP, respectively.
They are in good agreement with the new bands detected
experimentally in the shorter wavelength region after
adding TFA. Therefore, these bands are confirmed to be ab-
sorption bands of the protonated forms. The TDDFT results
show that all the absorption maxima correspond to transi-
tions from HOMO to LUMO. In the HOMO and LUMO of
the protonated compounds (Figure 12), the LE character of
the transition from HOMO to LUMO is distinctly evident.
Figure 13. Transition density (TD) and charge difference density (CDD)
of QTCP in planar and twisted conformations. Green and red stand for
hole and electron, respectively. The isovalue is 4”10À4 a.u.
dipole moment of S2 is smaller than that of S1 can be easily
understood.
Large changes in transition dipole moment and polariza-
bility in the transition process may result in a nonlinear opti-
cal (NLO) response, which is very important for two-photon
absorption in the donor–acceptor system.[76–78] They can be
fitted by the static electric field dependent transition energy
Eexc(F)=Eexc(0)ÀDmFÀDaF2/2, where Eexc(0) is the excita-
tion energy at zero field F, Dm the change in dipole moment,
and Da the change in polarizability.[78] According to the
fitted results shown in Figure 14, for single-photon absorp-
tion, the absorption peak of S1 should be larger than that of
Figure 12. HOMO and LUMO for protonated compounds in planar con-
formation.
2D and 3D real-space analysis of QTCP: Transition energies
and oscillator strengths were calculated with the TD-B3LYP
method and the 6-31G(D,P) basis set by using the Gaussi-
G
an03 program suite. Transition density (TD) and charge dif-
ference density (CDD) for QTCP at the planar and twisted
conformations are shown in Figure 13. The charge difference
densities clearly reveal the result and orientation of ICT.
The electron and hole populations of the S1 state are well
separated, that is, the electron can transfer from the nitro-
gen atom of the donor and benzene ring to the thiophene
ring and the cyano group. This is accordance with our analy-
sis above. The orientation and strength of the transition
dipole moments of the excited states can be determined
from the TDs. Moreover, the S2 state has two transition
dipole moments with opposite orientations (m=ma +mb).
Figure 14. Excitation energy versus electric field strength for the S1 and
S2 states. Insets show the fitted results.
2
Given the relationship jmj ~f/E,[75] the reason why the
6944
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Chem. Eur. J. 2008, 14, 6935 – 6947

Thiophene-p-Conjugated Donor–Acceptor Systems
FULL PAPER
The synthesis of the D-p-A compounds involved in the present work is
shown in Scheme 1. Starting compound 1 was synthesized according to a
literature procedure.[79,80] Wittig–Horner reactions of 1 and corresponding
phosphonates gave QT and 2 in high yield. Introduction of the aldehyde
group on the thiophene ring with by nBuLi and DMF provided QTC.
Compound 3 was obtained by Vilsmeier reaction of 2. Compounds
QTCP and QBCP were synthesized by Knoevenagel condensation of the
corresponding aldehydes with diethyl cyanomethylphosphonate and pi-
peridine as catalyst. All intermediates and target compounds were char-
S2, since m1 >m2, and for two-photon absorption the peak of
S2 would be larger than S1, since a2 @a1.[78]
Conclusion
The newly synthesized thiophene-p-conjugated D-p-A com-
pounds QT, QTC, QTCP, and QBCP have been systemati-
cally investigated by experimental and theoretical methods.
Both steady-state spectroscopic results and theoretical calcu-
lations show that the fluorescence emission of the thio-
phene-p-conjugated compounds is strongly dependent on
the electron-withdrawing substituents at the thienyl 2-posi-
tion. Compound QTCP can emit both S1 and S2 fluores-
cence. For QTCP in nonpolar solvents, S1 fluorescence is
emitted from the planar conformation, whereas S1 fluores-
cence is emitted from the twisted ICT (TICT) state in the
perpendicular conformation for QTCP in polar solvents.
Furthermore, the TICT mechanism of QTCP in polar sol-
vents has been demonstrated by various spectroscopic re-
sults and theoretical calculations. Compound QTCP can
also emit S2 fluorescence in both nonpolar and polar sol-
vents in the planar conformation, and S2 fluorescence was
confirmed by the fluorescence excitation spectra. The S1
fluorescence behavior of QTC in different solvents is similar
to that of QTCP. However, no S2 fluorescence can be ob-
served for QTC in both nonpolar and polar solvents. Com-
pound QT also exhibited no S2 fluorescence in various sol-
vents. Our theoretical calculations showed that the energy
gaps between the S2 and S1 states for QTC and QT are
markedly smaller than that of QTCP. The small energy gap
between S2 and S1 states facilitates nonradiative deactivation
of the S2 state, and thus S2 fluorescence is quenched. More-
over, our spectroscopic results show the absence of a TICT
state for QT and QBCP. This was demonstrated by our cal-
culated potential-energy curves of these compounds. There-
fore, the S1 fluorescence can only be emitted from the
planar conformation for QT and QBCP in various solvents.
Interestingly, strong S2 fluorescence can be observed for
QBCP in polar solvents. This is attributed to the larger
energy gap between S2 and S1 states of QBCP compared to
QTCP.
1
acterized by H NMR spectroscopy and HRMS.
1
QT: H NMR ([D6]acetone, 400 MHz): d=1.21 (s, 3H), 1.30 (s, 3H), 1.36
(d, J=6.7 Hz, 3H), 1.46 (dd, J1 =12.7 Hz, J2 =13.0 Hz, 1H), 1.85 (dd, J1 =
4.5 Hz, J2 =12.9 Hz, 1H), 2.84 (s, 3H), 2.80–2.87 (m, 1H), 6.56 (d, J=
8.5 Hz, 1H), 6.88 (d, J=16.0 Hz, 1H), 6.98–7.0 (m, 1H), 7.04 (d, J=
3.5 Hz, 1H), 7.15 (d, J=16.1 Hz, 1H), 7.23–7.24 (m, 2H), 7.31 ppm (s,
1H); HRMS-EI calcd for C19H23NS [M]+: 297.1551; found: 297.1546.
QTC: 1H NMR ([D6]acetone, 400 MHz): d=1.24 (s, 3H), 1.32 (s, 3H),
1.37 (d, J=6.6 Hz, 3H), 1.47 (dd, J1 =12.7 Hz, J2 =12.9 Hz, 1H), 1.87 (dd,
J1 =4.4 Hz, J2 =13.0 Hz, 1H), 2.88 (s, 3H), 2.79–2.85 (m, 1H), 6.59 (d, J=
8.6 Hz, 1H), 7.20–7.22 (m, 3H), 7.33 (dd, J1 =1.7 Hz, J2 =8.5 Hz, 1H),
7.39 (s, 1H), 7.82 (d, J=3.9 Hz, 1H), 9.85 ppm (s, 1H); HRMS-EI calcd
for C20H23NOS: 325.1500 [M]+; found: 325.1500.
QTCP: 1H NMR ([D6]acetone, 400 MHz): d=1.24 (s, 3H), 1.33 (s, 3H),
1.34–1.39 (m, 9H), 1.47 (dd, J1 =12.9 Hz, J2 =13.0 Hz, 1H), 1.87 (dd, J1 =
4.4 Hz, J2 =13.1 Hz, 1H), 2.80–2.89 (m, 4H), 4.14–4.21 (m, 4H), 6.60 (d,
J=8.5 Hz, 1H), 7.24–7.25 (m, 3H), 7.37 (d, J=8.5 Hz, 1H), 7.44 (s, 1H),
7.79 (d, J=4.0 Hz, 1H), 8.03 ppm (d, J=19.2 Hz, 1H); HRMS-EI calcd
for C26H33N2O3PS: 484.1949 [M]+; found: 484.1953.
QBCP: 1H NMR ([D6]acetone, 400 MHz): d=1.20 (s, 3H), 1.29 (s, 3H),
1.31–1.36 (m, 9H), 1.38 (dd, J1 =12.9 Hz, J2 =13.0 Hz, 1H), 1.76 (dd, J1=
12.8 Hz, J2=4.2 Hz, 1H), 2.56–2.64 (m, 1H), 2.86 (s, 3H), 4.10–4.14 (m,
4H), 6.43 (d, J=8.8 Hz, 1H), 6.79 (s, 1H), 6.95 (d, J=8.7 Hz, 1H), 7.27–
7.34 (m, 2H), 7.46–7.52 (m, 4H), 7.79 ppm (d, J=20 Hz, 1H); HRMS-EI
calcd for C28H35N2O3P: 478.2385 [M]+; found: 478.2387.
Photophysical properties of QT, QTC, QTCP, and QBCP were also in-
vestigated by TDDFT calculations, which were performed with the TUR-
BOMOLE program suite.[81] The TDDFT method is widely used to calcu-
late electronic excitation spectra with analytical gradient implementa-
tions permitting excited-state geometry optimizations.[8,14,82] Both the
geometric structures of ground state and the low-lying electronically ex-
cited states were optimized at the B3LYP level with a basis set of triple-z
valence quality and one set of polarization functions (TZVP).[83,84] Fine
quadrature grids of size 4 (both for ground state and excited state) were
employed.[85] For the purpose of comparison, the electronic structures of
all TMTHQs were also calculated by DFT with B3LYP functional and 6-
[86]
31G(D,P) basis set by using the Gaussian03 program package. The
E
transition energies and oscillator strengths were calculated by TDDFT
with B3LYP functional and 6-31G(D,P) basis set. The 2D site and 3D
AHCTREUNG
cube representations used in the present study have been described in
detail elsewhere.[87–92] Briefly, the 3Dtransition density reveals the orien-
tation and strength of the transition dipole moment, and the 3Dcharge
difference density shows the orientation and results of ICT. The 2Dcon-
tour plot of the transition density matrix reveals the electron–hole coher-
ence and magnitudes of delocalization (along the diagonal) and exciton
(along the off-diagonal).[88]
Experimental Section
1H NMR spectra were obtained on a Varian INOVA 400 MHz NMR
spectrometer. Mass spectra were recorded on a Q-TOF mass spectrome-
ter (Micromass, England). The electronic absorption spectra were mea-
sured on a HP-8453 spectrophotometer. The fluorescence measurements
were performed on a PTI-C-700 Felix and Time-Master system. The fluo-
rescence quantum yields were determined by the relative method using
optically matched solutions. Quinine sulfate in 1n sulfuric acid (Ff =
0.546 at 258C) was used as standard. The accuracy of the quantum yields
reported here is expected to be better than ꢀ10%. Fluorescence life-
times were determined on a chronos fluorescence lifetime spectrometer
(ISS Champagn, IL, USA). Solvents were used as received for absorption
and fluorescence spectral measurement.
Acknowledgement
This work was supported by the National Natural Science Foundation of
China (Nos. 20373071, 20333050, and 20403020), and NKBRSF
(2007CB815202), the Swedish Research Council, and the K & A Wallen-
berg Foundation.
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Received: November 27, 2007
Revised: April 24, 2008
Published online: June 24, 2008
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6947