Fluorescence self-quenching and charge transfer of a novel A-σ-π-σ-A type of silicon-bridged compound

Article abstract of DOI:10.1016/j.jphotochem.2010.07.002

A novel A-σ-π-σ-A type of silicon-bridged compound, 1,4-bis(cyanophenyldimethylsilyl)benzene (CPDMB), was synthesized and fully characterized by FT-IR, ¹H NMR, ¹³C NMR, HRMS, and single crystal X-ray diffraction. Ultraviolet properties and steady-state fluorescence of CPDMB were studied. The concentration self-quenching phenomena of CPDMB were found in 2-propanol, THF, and CH₂Cl₂. It can be well interpreted by photoinduced self-assembly behavior (J-aggregate) of the CPDMB in solvents. Moreover, the effects of solvents on the fluorescence self-quenching phenomena were also investigated. In order to interpret the novel photoelectric properties of CPDMB, DFT calculations on gas phase CPDMB were carried out.

Full text of DOI:10.1016/j.jphotochem.2010.07.002

Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 241–247
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Fluorescence self-quenching and charge transfer of a novel A-␴-␲-␴-A type of
silicon-bridged compound
Peng Wang, Sheng-Yu Feng∗
Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University,
Shanda South Road 27, Jinan 250100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
novel A-␴-␲-␴-A type of silicon-bridged compound, 1,4-bis(cyanophenyldimethylsilyl)benzene
(CPDMB), was synthesized and fully characterized by FT-IR, 1H NMR, 13C NMR, HRMS, and single crystal
X-ray diffraction. Ultraviolet properties and steady-state fluorescence of CPDMB were studied. The con-
centration self-quenching phenomena of CPDMB were found in 2-propanol, THF, and CH2Cl2. It can be
well interpreted by photoinduced self-assembly behavior (J-aggregate) of the CPDMB in solvents. More-
over, the effects of solvents on the fluorescence self-quenching phenomena were also investigated. In
order to interpret the novel photoelectric properties of CPDMB, DFT calculations on gas phase CPDMB
were carried out.
Received 12 March 2010
Received in revised form 7 June 2010
Accepted 2 July 2010
Available online 31 July 2010
Keywords:
Silicon-bridged compound
Fluorescence self-quenching
Charge-transfer
© 2010 Elsevier B.V. All rights reserved.
J-aggregate
1. Introduction
erate inclusion of conjugation-interrupting units shows potential
to alter and improve the optical and photophysical proper-
Charge transfer is a characteristic feature of organic molecules
where electron donor (D) and/or acceptor (A) groups are linked
by ␲-conjugated bridges with sp- or sp2-carbon structural units
such as D-␲-A [1,2], D-␲-A-␲-D or A-␲-D-␲-A [3–8], D-␲-D [9–11]
and A-␲-A [12] types. Such organic molecules were intensively
studied both theoretically and experimentally, due to their novel
photoelectric properties, such as nonlinear-optical behavior [8],
photon absorption properties [13], solvatochromism [1–12], and
so on.
ties of polymers [18–21]. In addition, diphenylanthracene [22]
and anthracene [23] moieties between silanylene spacers have
been introduced into molecules to obtain organic light-emitting
diodes (OLED) and to study photoinduced energy transfer, respec-
tively.
As classical acceptors, cyanoaryl groups can be used to
construct photosensitive compounds [24]. They were also intro-
duced into side-chain of polymers to trace the process of
photoinduced polymerization [25]. According to our previ-
ous calculations [26], introducing of the cyano substitution to
bis(2-thienyl)dimethylsilane can lead to the evident increase of
molecular polarity and electrophilicity. In addition, attributed
to conformational nonrigidity, manageable properties, and steric
effects of the silicon atom, such p-disilanyl-phenyl-bridging com-
pounds as 1,4-bis(dimethyl-4-pyridylsilyl)benzene was prepared
and used to construct supramolecular framework with novel
channels [27]. However, less photoelectrical studies of p-disilanyl-
phenyl-bridging compounds were reported.
In this paper, in order to study the photoinduced charge-
transfer mechanisms of p-disilanyl-phenyl-bridging compounds,
1,4-bis(cyanophenyldimethylsilyl)benzene (CPDMB), as a novel
A-␴-␲-␴-A type of compound, was synthesized and fully charac-
terized. Photoinduced charge-transfer mechanism of CPDMB and
corresponding fluorescence self-quenching behavior in solutions
were investigated. In order to interpret the novel photoelectric
properties of CPDMB, DFT calculations on gas phase were carried
out.
Although the sp3-silicon unit is generally a poorer structural
motif than sp- or sp2-carbon groups in facilitating intramolecu-
lar electronic communication, conjugated compounds with the
silanyl or disilanyl spacers as ␴-bridges have been systemati-
cally studied. Sakurai and coworkers’ investigation on arylsilanes
demonstrated that Si–Si bonds and aryl groups function as electron
donors and acceptors, respectively [14]. Zyss and co-workers stud-
ied the second-order NLO activity of donor–acceptor substituted
silanes, which they attributed to the presence of intramolecu-
lar CT (ICT) interactions [15,16]. A study by van Walree and
coworkers indicated that a single silanyl spacer between two
conjugated moieties can facilitate the ICT process [17]. Subse-
quently, Luh et al. introduced an alternating silanylene spacer as
an insulator into alternating co-polymers, which proved that delib-
∗
Corresponding author. Tel.: +86 531 8836 4866; fax: +86 531 8856 4464.
E-mail address: fsy@sdu.edu.cn (S.-Y. Feng).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.07.002

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P. Wang, S.-Y. Feng / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 241–247
2. Experimental
2.1. General procedures and materials
All operations were carried out under dry argon using stan-
dard Schlenk techniques. 4-Bromobenzonitrile (Alfa) and other key
reagents including n-butyllithium were used as received. The com-
pound 1,4-ClMe2SiC6H4SiMe2Cl was prepared according to the
literature procedures [28]. THF and diethyl ether were distilled
from sodium/benzophenone ketyl immediately prior to use in all
experiments. The other organic solvents were dried and purified
according to standard procedures. Column chromatography was
performed on silica gel (200-300 ASTM).
NMR spectra were obtained on a Bruker AC-300/AC-400 NMR
spectrometer (1H, 300.1 MHz or 400.1 MHz; 13C, 75.5 MHz). CDCl3
was used as the solvent for all of the products. Chemical shifts
(ppm) were determined relative to internal CHCl3 (1H, ı 7.26),
internal CDCl3 (
13C, ı 76.3). FT-IR spectra were obtained in the
400–4000 cm−1 range using a Bruker TENSOR-27 FTIR Spectrom-
eter. Mass spectra were recorded on an Agilent Q-TOF6510 Mass
Spectrometer. Solution UV spectra were measured using a HITACHI
U-4100 spectrophotometer in spectrophotometric grade solvents.
Solid state UV spectra were measured on a Shimadzu UV-2500
spectrophotometer. The steady-state fluorescence spectra were
recorded with a PerkinElmer LS 55 spectrofluorimeter. The emis-
sion lifetime measurements were carried out on FL920 fluorescence
lifetime spectrometer. X-ray crystallography was performed on a
Bruker Smart 1000 diffractometer at room temperature (25 ◦C).
Fig. 1. Molecular structure of CPDMB and selected bond distances (Å) and
angles (deg): Si1–C1 1.884(3), Si1–C8 1.867(3), Si1–C10 1.893(3), C7–N1 1.150(3);
C9–Si1–C8 112.0(2), C9–Si1–C1 108.03(15), C9–Si1–C10 110.68(15), C6–C1–C2
115.9(2), C3–C2–C1 123.1(3), and N1–C7–C4 178.2(3).
2.2. Synthesis of CNC6H4Me2SiC6H4SiMe2C6H4CN (CPDMB)
To n-BuLi (5mmol) in hexanes/diethyl ether (2/10 mL) mixed
solvents, a solution of 4-bromobenzonitrile (0.91 g, 5 mmol) in
THF (10 mL) was added at −78 ◦C [29]. After addition, the mix-
ture was kept at −78 ◦C for 30 min, and then a solution of
4-ClMe2SiC6H4SiMe2Cl (0.60 g, 2.5 mmol) in THF (10 mL) was
added dropwise. After 2 h, the mixture was warmed to room tem-
perature and then quenched by appropriate amount of water. The
volatiles were removed from the resulting mixture by vacuum
distillation. The residue was purified by column chromatography
using dichloromethane/n-hexane following by ethyl acetate as elu-
ent to afford 0.80 g of CNC6H4Me2SiC6H4SiMe2C6H4CN as a white
solid (80.8%). IR (cm−1) (KBr): 2224 (CN), 1252, 823, 788 (Si–Me),
1099 (Si–Ar), 1824, 1929 (central Ar); 1H NMR, ı 0.59(s, 12H, MeSi),
7.50(d, 8H, Ph–H/Si-␤-H), 7.63(d, 4H, Ph–H/Si-␥-H); 13C NMR, ı
−2.85(MeSi), 118.91 (CN), 112.81, 131.07, 133.59, 134.59, 138.16,
144.96 (Ph–C); MS (m/z): 397.15 [M+H]+, 397.15 (Anal. Calcd.).
geometry of CPDMB was optimized in gas phase by a standard
force-minimization procedure.
3. Results and discussion
3.1. Synthesis
Limited by rigorous temperatures for lithiation of halogenated
benzonitriles [33], cyanophenyl silanes were prepared generally
through reaction of bromophenylsilane and copper(I) cyanide [17].
The latest paper [29] reported that mixed solvents (THF/diethyl
ether) and reverse addition from bromobenzonitriles to n-BuLi
can increase the yield of lithiated benzonitriles. According to such
optimized method, we prepared the target molecule CPDMB with
p-disilanyl-phenyl-bridge in higher yield (80.8%) by the reactions of
4-lithiobenzonitrile with 4-ClMe2SiC6H4SiMe2Cl. The experiments
show that the method used in this work provides competitive
yield of cyanophenyl silanes without heavy metallic salt. Similarly,
the reference compound DCPDM was also obtained successfully
according to this procedure.
2.3. Synthesis of CNC6H4SiMe2C6H4CN (DCPDM)
The colorless crystal of di(4-cyanophenyl)dimethylsilane
(DCPDM) was prepared by the reaction of n-BuLi, 4-
bromobenzonitrile and dimethyldichlorosilane in accordance
with the same procedures by using Me2SiCl2 instead of 4-
ClMe2SiC6H4SiMe2Cl. The yield was 35.5%. IR (cm−1) (KBr): 2223
(CN), 1255, 850–780 (Si–Me), 1100 (Si–Ar); 1H NMR, ı 0.63 (s, 6H,
MeSi), 7.59–7.61 (d, 4H, Ph–H), 7.65–7.67 (d, 4H, Ph–H); 13C NMR,
ı −3.05 (MeSi), 118.63 (CN), 113.37, 131.28, 134.53, 143.45 (Ph–C).
3.2. Molecular structure of CPDMB
The molecular structure of CPDMB has been determined by
single-crystal X-ray diffraction (Fig. 1 and Table 1). It crystallizes in
space group (P21/n). The single molecule has a crystallographically
imposed centre of inversion, and two molecules fill the monoclinic
unit cell.
2.4. Quantum chemistry calculations
In the crystalline state of CPDMB, the planes of benzene rings
in the two cyanophenyl moieties are parallel to each other. The
plane of benzene ring in p-disilanyl-phenyl-bridging unit is nearly
perpendicular (79.5◦) to these two planes although the angles
around silicon atoms (there into C9–Si1–C1, 108.03◦; C1–Si1–C10,
107.26◦; C8–Si1–C1, 109.16◦; C8–Si1–C10, 109.56◦) follow the
To gain insight into the electric and optical properties of CPDMB,
DFT calculations in gas phase were performed using the Gaus-
sian 03 program package [30]. Hybrid B3LYP exchange-correlation
functional [31] and 6-31+G basis set [32] were employed in
consideration of both accuracy and efficiency. The ground-state

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243
Table 1
excited singlet electronic states of aromatic system. The same as
discussion of di(4-cyanophenyl)dimethylsilane (DCPDM) [17], the
shorter wavelength bands named 1La are predominantly based on
the promotion of an electron from the highest-occupied molec-
ular orbital (HOMO) to the second-lowest-unoccupied molecular
orbital (SLUMO) named S2 mechanism. The longer wavelength
bands named 1Lb correspond to a HOMO–LUMO transition (S1
mechanism). Compared with analogous systems such as 1,1ꢀ-
bicyclohexyliden-4-ylidenepropanedinitrile [35], the 237 nm can
also be assigned to ␲ → ␲* transition of cyano acceptor, and the
weaker band at 271/274/277 nm can be ascribed to a transition
involving charge transfer from the benzene ring to cyano acceptor.
In absorption spectra of different concentrations of CPDMB in the
three solvents, no obvious change of band shape was found. Besides,
no obvious ␴-to-␲ charge-transfer absorptions over 300 nm were
found in the spectra, indicating that the p-disilanyl-phenyl-bridge
in CPDMB does not serve as conductor but insulator for chro-
mophores in ground state. The through-bond interaction of the two
cyanophenyl groups was forbidden in ground state.
Crystallographic data for CPDMB.
CPDMB
Empirical formula
Fw
Cryst syst
Space group
a, Å
b, Å
c, Å
˛, deg
ˇ, deg
ꢀ, deg
V, Å3
Z
Dc, g cm−3
No. of rflns collected
No. of unique data
Rint
C24H24N2Si2
396.63
Monoclinic
P21/n
12.721(10)
6.154(5)
15.109(11)
90.00
103.610(13)
90.00
1149.6(15)
2
1.14576
5092
1869
0.0413
ꢁmax, deg
R1 (I > 2ꢂ(I))
wR2 (all data)
24.31
0.0477
0.0691
3.4. Ground state excitation and emission fluorescence
rules of tetrahedron character. Moreover, the angles C6–C1–C2
(115.9◦) and C11–C10–C12 (115.2◦) are smaller, while the angles
C5–C6–C1 (122.3◦), C3–C2–C1 (123.1◦), C10–C11–C12A (122.2◦),
and C10–C12–C11A (122.3◦) are bigger indicating that the config-
urations of benzene rings were influenced by the ␴–␲ interaction
of silicon-spacer and benzene rings. The slightly bent cyano group
(N1–C7–C4 = 178.2◦) is nearly on the plane of the benzene ring in
cyanophenyl moiety which can provide the excellent conjunction
for the intramolecular charge-transfer (IACT).
The ground state excitation spectra and photoluminescence
spectra of CPDMB were characterized in 7 different solvents, n-
Bu2O, 2-propanol, THF, chloroform, methanol, DMSO, and CH2Cl2
(Fig. 8 in SI). The shifts of excitation bands of CPDMB in different
solvents are more obviously than emission ones. Obviously, there
are no solvent effects on the fluorescence bands to be observed
at 307 nm in 6 different solvents except DMSO (Fig. 8(B) in SI).
In contrast, the red shift of the broad fluorescence band at longer
wavelength in DMSO was observed, indicating that the emitting
state are polar, which can be treated as the orthaonol intramolecu-
lar charge transfer (OICT) state occurring by stabilizing the CT states
in polar solvents, as reported on aryldisilanes [36]. Whereas, the
stocks shifts of CPDMB are not solvent-polarity dependent, which
means weak IACT in CPDMB molecule even in excited state.
3.3. Electronic absorption spectra
Electronic absorption spectra of CPDMB in 2-propanol, THF,
CH2Cl2, and solid state are shown in Fig. 2. Maximum absorp-
tion wavelength and corresponding molar extinction coefficients of
CPDMB and reference compound DCPDM are compiled in Table 2.
No additional band was found in absorption spectra of CPDMB
compare to those of DCPDM. Whereas, compared with the DCPDM
in 2-propanol, the higher molar extinction coefficients of CPDMB
were found. Two bands at 237 nm and 271 nm (in 2-propanol),
274 nm (in THF), and 277 nm (in CH2Cl2) (Fig. 2) can be referred
to 1La and 1Lb bands of CPDMB, which were labeled by Platt
[34] for different transitions from the ground states to different
3.5. Self-quenching behavior
The concentration dependent excitation and emission spectra of
CPDMB were found in THF and 2-propanol (see Fig. 3). As shown in
Fig. 2(A) and (C), the excitation bands at 237 nm lower and excita-
tion bands at 274/271 nm grow with concentration of CPDMB from
1.0 × 10−5 to 4.0 × 10−4 mol L−1. Also, the original excitation bands
at 237 nm were red shifted following the increase of concentration
in certain range.
For intensive study of the influences from the excitation spec-
tra of solvents, the excitation spectra of CPDMB in solid state,
THF, and 2-propanol were characterized, separately. Two excited
bands at 218 and 248 nm of CPDMB in solid state were found
(Fig. 9(A) in SI). It means that there are nearly 30 nm red-shifts
of excited bands of CPDMB in both solvents. Also some shoulders
of excitation band were found at longer wavelength, which means
the aggregation of monomer in solid state. To study the interac-
tion of solvents and solute, the excitation bands of 2-propanol and
THF were found at 250 and 257 nm, separately (Fig. 9(B) in SI),
which can be attributed to n–␴* electron transitions of O atoms
with lone-pair electrons in molecular structures [37]. There are
three major interactions between cyanophenyl group and solvent
molecule: (1) dipole–dipole interaction, (2) a hydrogen-bond-like
interaction that the protic solvent partially donates hydrogen atom
to the ␲-cloud of the nitrile triple bond, and (3) a second hydrogen-
bonding interaction between one of the ring-hydrogen atoms and
the lone pair electrons of the O or N atom in the solvent molecule
[38,39]. However, the same phenomenon was also found in CH2Cl2
(Fig. 10 in SI), which indicates that neither (2) nor (3) is the defini-
Fig. 2. Solution (9.0 × 10−5 M) absorption spectra of CPDMB in 2-propanol “—”, THF
“—”, CH2Cl2 “. . .”, and the solid state absorption spectrum of CPDMB “- -”.

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P. Wang, S.-Y. Feng / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 241–247
Table 2
The ꢃmax (nm) and extinction coefficient (ˇ, 10−3 mol−1 L cm−1) of CPDMB and DCPDM.
Compounds
CPDMB
DCPDM
ꢃ1 (ε1)
ꢃ2 (ε2)
ꢃ1 (ε1)
ꢃ2 (ε2)
THF
2-Propanol
CH2Cl2
237 (37.6)
237 (64.8)
237 (76.0)
274 (2.29)
271 (4.03)
277 (6.39)
–
–
237 (26.3)
–
279 (1.52)
–
tive factor for concentration dependent excitation of the CPDMB.
The corresponding emission spectra of CPDMB in 2-propanol
(Fig. 2(B)), THF (Fig. 2(D)), and CH2Cl2 (Fig. 10(B, C) in SI) were also
shown. Clearly, the fluorescence excited at 237 nm was quench-
ing, and fluorescence excited at 274/271 nm was strengthened
with the increase of concentration. The linearly concentration
dependence of the fluorescence intensities in 2-propanol, THF, and
CH2Cl2 are shown in Fig. 4(A). Obviously, the Stern–Volmer plots of
quenching fluorescence (Fig. 4(B)) cannot be fitted to linear, which
means that the fluorescence excited at 237 nm is quenched by not
only monomer, but also aggregator. The enhanced fluorescence
intensity excited at longer wavelengths shows that the quenching
energy is mainly released by radiation, indicating that the quench-
ing was not caused through collision but through charge-transfer
or energy-transfer mechanism between molecules. Moreover, the
emission lifetimes of CPDMB in 2-propanol at different concentra-
tions (1 × 10−6, 1 × 10−4) were measured (Fig. 11 in SI). Both excited
wavelengths at 237 and 271 nm at two concentrations were used
to measure the emission lifetimes. Although the emission lifetime
excited at 271 nm cannot be got for weak emissions, the other three
lifetimes were measured to be 6.65 ns (1 × 10−6, 237 nm), 6.58 ns
(1 × 10−4, 237 nm), and 6.51 ns (1 × 10−6, 271 nm). Consequently,
the decrease of lifetime was not obviously (∼0.1 ns) following the
aggregation behavior, which can be interpreted by concomitant
monomer species at high concentrations.
Generally, donor–acceptor systems with cyanophenyl as an
acceptor group always show novel and classic photoelectric prop-
erties such as IACT [40]. However, according to solvent-polar
Fig. 3. Excitation and fluorescence spectra of CPDMB in solutions at room temperature with the concentration at (a) 1.0 × 10−5 M, (b) 3.0 × 10−5 M, (c) 5.0 × 10−5 M, (d)
7.0 × 10−5 M, (e) 9.0 × 10−5 M, and (f) 4.0 × 10−4 M. 2-propanol: (A) Excitation spectra with the emission at 300 nm. (B) Fluorescence spectra excited at 237 nm and 271 nm.
THF: (C) Excitation spectra with the emission at 300 nm. (D) Fluorescence spectra excited at 237 nm and 274 nm.

P. Wang, S.-Y. Feng / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 241–247
245
Fig. 4. (A) Fluorescence intensities (ꢃem = 300 nm) of CPDMB in 2-propanol (red), THF (black), CH2Cl2 (blue) at different concentrations: solid points (ꢃex = 237 nm) and
hollow points (ꢃex = 271/274/274 nm); (B) Stern–Volmer plots of CPDMB self-quenched in 2-propanol (red), THF (black), CH2Cl2 (blue) (ꢃex = 237 nm, ꢃem = 300 nm). (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
independent stead-state fluorescence properties of the CPDMB,
the through-bond interaction of the two cyanophenyl moieties is
weak. It indicates that the IACT between two cyanophenyl groups
in CPDMB molecule is forbidden by p-disilanyl-phenyl-bridge. So
the charge transfer or energy transfer of the CPDMB molecules
occur through-space instead of through-bond. As reported by Li
et al. [25], fluorescence “structural self-quenching effect” (SSQE)
can be observed for polymers with vinyloxy monomer bearing
cyanophenyl moiety. The SSQE was considered to be ascribed to
the formation of a charge transfer exciplex between the electron-
accepting chromophore and the coexisting electron-donating
carbon–carbon double bond upon UV light irradiation. For inter-
preting SSQE, the formations of exciplex between the cyanophenyl
and the vinyloxy group upon UV irradiation as well as mutual inter-
action between adjacent cyanophenyl were described in Scheme 1.
In our systems, due to the steric hindrance effects in molecule and
structural properties of silicon atoms, two cyanophenyl groups can-
not locate at the same side of central-phenyl-ring-plane but locate
at both sides of central-phenyl-ring-plane, and they even parallel
each other as shown in crystal structure (Fig. 1). So through-space
intramolecule interaction of the two cyanophenyl moieties was for-
bidden. Therefore, formation of intermolecular exciplex between
CPDMB molecules is most possible. In addition, crystal structure
stacking can be treated as structure of aggregates (N = ∞). Also,
compared with H-aggregates, J-aggregates in particular are very
seldom observed because only a few molecules form J-aggregates
have been clearly identified [41]. And concentration-dependent
excitation of CPDMB in solvents at certain concentration range
(1.0 × 10−5–9.0 × 10−5 M) proved the aggregation of the CPDMB.
As shown in Fig. 3(A) and (C), the excitation band at shorter wave-
length was gradually red shifted with the increase of concentration,
and the excitation band at longer wavelength gradually grew at
the same time. So it can be attributed to J-aggregation according
to Kasha’s exciton theory [42]. The relation of band shifts with
concentration follows:
2
2(N − 1)
ꢄꢅ =
(1 − 3 cos2 ˛)
(1)
hcNr3
where the ꢄ␯ is the separation between wavenumbers of aggre-
gate and monomer, h is Planck constant, c is the velocity of light,
is transition (dipole) moment, r is the distance between two
dipoles, ˛ is the angle made by the polarization axes of the unit
molecule with the line of molecular centers. When ˛ < 54.7◦, the
molecule will be J-aggregate; when ˛ > 54.7◦, the molecule will
be H-aggregate. Although some parameters in equation cannot be
obtained easily, the type of aggregation can be confirmed by value
of ˛ angle. In the crystal packing structures of CPDMB, acceptor
sites on neighboring molecules are oriented parallel to each other.
To obtained the value of ˛ angle, the center-to-center distance of
benzene rings (rcc) among CP of neighboring molecules was con-
firmed to be 4.846 Å, and the distances of two CP-planes (rpp) was
confirmed to be 3.562 Å. So ˛ angle can be confirmed to be 47.3◦
according to Eq. (2):
rpp
sin ˛ =
(2)
rcc
So the concentration self-quenching behavior of CPDMB can be
attributed to J-aggregation of the molecules in higher concentra-
tions (>10−5 mol L−1), which can be represented in the model and
corresponding crystal structure stacking picture (Fig. 5). In addi-
tion, gradual growth of the excitation band at longer wavelength
can be attributed to intermolecular charge transfer (IECT) from the
benzene ring of cyanophenyl to the cyano acceptor of neighboring
molecule.
Scheme 1. Exciplex model of SSQE [15].

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P. Wang, S.-Y. Feng / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 241–247
Fig. 5. Exciplex model of J-aggregate of CPDMB (A) and corresponding crystal structure stacking picture (B).
spread the whole molecule, which was also reported for DCPDM
[17]. Whereas, these processes increase the density of the
electron around cyanophenyl groups, which help to enhance
the IECT between adjacent cyanophenyl groups. So the con-
centration self-quenching fluorescence of CPDMB can be well
interpreted by enhanced IECT between J-type aggregated CPDMB
molecules.
4. Conclusions
An A-␴-␲-␴-A type of silicon-bridged compound, 1,4-
bis(cyanophenyldimethylsilyl)benzene (CPDMB), was synthesized
and characterized by FT-IR, 1H NMR, 13C NMR, HR-MS, and
single-crystal X-ray diffraction. The novel concentration
self-quenching phenomena in different solvents at higher
concentrations (>10−5 mol L−1
) were found. The phenomena
can be well interpreted by the photoinduced self-assembly (J-
aggregation) of CPDMB in solvent, which was also shown in crystal
structure stacking picture. According to solvent-independent
stocks shifts and frontier bonds analysis, IACT between two
cyanophenyl groups in CPDMB is nearly forbidden. Moreover,
donor ability and structural effects of p-disilanyl-phenyl-bridge
in molecule enhance the IECT of CPDMB. IECT of CPDMB in sol-
vents open up for a new charge-transfer type of silicon-bridged
compounds.
Fig. 6. Frontier orbitals calculated for CPDMB in the gas phase at the DFT level
(isodensity = 0.02 au).
3.6. Theoretical calculations
In order to interpret the novel photoelectric properties of
CPDMB, DFT calculations were carried out. The frontier orbitals
contributing to the electronic transition were sketched in Fig. 6,
the HOMO mainly locates on the benzene ring of p-disilanyl-
phenyl-bridge and partly on the cyanophenyl groups (acceptor),
Supplementary material
and LUMO shows
a significant density on the cyanophenyl
FT-IR spectra, 1H NMR spectra, 13C NMR spectra of CPDMB and
DCPDM; HRMS spectra, Crystallographic Data, and fluorescence
spectra (in CH2Cl2) of CPDMB were given. Crystallographic data
(excluding structure factors) for the structures reported have been
deposited with the Cambridge Crystallographic Data Center as sup-
plementary publication no. CCDC-758060 for CPDMB, and can be
obtained free of charge on application to the CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK. [Fax: Internet C44 1223/336 033 E-mail:
deposit@ccdccam.ac.uk].
groups of CPDMB and some on the benzene ring of p-disilanyl-
phenyl-bridge. Moreover, there is less density of delocalized
electrons on the benzene ring of p-disilanyl-phenyl-bridge at
the SLUMO level. It means that during the two excitation pro-
cesses (HOMO → LUMO and HOMO → SLUMO), the benzene ring
of p-disilanyl-phenyl-bridge and the two cyanophenyl groups
act as the electron-donor and electron-acceptors, respectively.
During the both processes, the electrons were almost local-
ized in both cyanophenyl groups, and delocalization can hardly

P. Wang, S.-Y. Feng / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 241–247
247
Acknowledgements
[19] Y.J. Cheng, T.Y. Hwu, J.H. Hsu, T.Y. Luh, Intrachain energy transfer in silylene-
spaced alternating donor–acceptor divinylarene copolymers, Chem. Commun.
(2002) 1978–1979.
[20] Y.J. Cheng, T.Y. Luh, Synthesis, Light-harvesting and energy-transfer properties
of regioregular silylene-spaced alternating [(Donor-SiMe2–)n=1–3–(Acceptor-
SiMe2)] copolymers, Chem. Eur. J. 10 (2004) 5361–5368.
The authors are grateful for financial support from the
National Natural Science Foundation of China (Nos. 20574043 and
20874057) and the Key Natural Science Foundation of Shandong
Province of China (No. Z2007B02).
[21] T.Y. Luh, Y.J. Cheng, Alternating divinylarene–silylene copolymers, Chem. Com-
mun. (2006) 4669–4678.
[22] T. Lee, K.H. Song, I. Jung, Y. Kang, S.H. Lee, S.O. Kang, J. Ko, Silylene-spaced
diphenylanthracene derivatives as blue-emitting materials, J. Organomet.
Chem. 691 (2006) 1887–1896.
Appendix A. Supplementary data
[23] H.W. Wang, Y.J. Cheng, C.H. Chen, T.S. Lim, W. Fann, C.L. Lin, Y.P. Chang,
K.C. Lin, T.Y. Luh, Thorpe-ingold effect on photoinduced electron transfer of
dialkylsilylene-spaced divinylarene copolymers having alternating donor and
acceptor chromophores, Macromolecules 40 (2007) 2666–2671.
[24] T. Scherer, W. Hielkema, B. Krijnen, R.M. Hermant, C. Eijckelhoff, F. Kerkhof,
A.K.F. Ng, R. Verleg, E.B. van der Tol, A.M. Brouwer, J.W. Verhoeven, Synthesis
and exploratory photophysical investigation of donor-bridge-acceptor systems
derived from N-substituted 4-piperidones, Recl. Trav. Chim. Phys-Bas. 112
(1993) 535–548.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.07.002.
References
[1] L. Beverina, J. Fu, A. Leclercq, E. Zojer, P. Pacher, S. Barlow, E.W. van Stryland,
D.J. Hagan, J.-L. Breˇıdas, S.R. Marder, Two-photon absorption at telecommuni-
cations wavelengths in a dipolar chromophore with a pyrrole auxiliary donor
and thiazole auxiliary acceptor, J. Am. Chem. Soc. 127 (2005) 7282–7283.
[2] L. Antonov, K. Kamada, K. Ohta, F.S. Kamounah, A systematic femtosecond study
on the two-photon absorbing D-␲-A molecules-␲-bridge nitrogen insertion
and strength of the donor and acceptor groups, Phys. Chem. Chem. Phys. 5
(2003) 1193–1197.
[3] S.-J. Chung, M. Rumi, V. Alain, S. Barlow, J.W. Perry, S.R. Marder, Strong,
low-energy two-photon absorption in extended amine-terminated cyano-
substituted phenylenevinylene oligomers, J. Am. Chem. Soc. 127 (2005)
10844–10845.
[4] M. Charlot, N. Izard, O. Mongin, D. Riehl, M. Blanchard-Desce, Optical limiting
with soluble two-photon absorbing quadrupoles: structure–property relation-
ships, Chem. Phys. Lett. 417 (2006) 297–302.
[5] M. Charlot, L. Porre‘s, C.D. Entwistle, A. Beeby, T.B. Marder, M. Blanchard-Desce,
Investigation of two-photon absorption behavior in symmetrical acceptor-
␲-acceptor derivatives with dimesitylboryl end-groups. evidence of new
engineering routes for TPA/transparency trade-off optimization, Phys. Chem.
Chem. Phys. 7 (2005) 600–606.
[25] F.S. Du, Z.C. Li, F.M. Li, Vinyl monomers bearing chromophore moi-
eties and their polymers. VIII. Synthesis and fluorescence behavior of
a
vinyloxy monomer having an electron-accepting chromophore moiety, p-
((vinyloxy)methyl)benzonitrile, and its polymers, J. Polym. Sci. Part A: Polym.
Chem. 37 (1999) 179–187.
[26] Y.Q. Ding, Q.A. Qiao, P. Wang, G.W. Chen, J.J. Han, Q. Xu, S.Y. Feng, A DFT study
of electronic structures of thiophene-based organosilicon compounds, Chem.
Phys. 367 (2010) 167–174.
[27] B. Park, I.S. Chun, Y.A. Lee, K.M. Park, O.S. Jung, Supramolecular framework with
two different kinds of channels via self-assembly of 1D coordination polymers
in a prismatic manner, Inorg. Chem. 45 (2006) 4310–4312.
[28] S.S.H. Mao, F.Q. Liu, T.D. Tilley, Efficient zirconocene-coupling of silicon-
substituted diynes to polymers and macrocycles, J. Am. Chem. Soc. 120 (1998)
1193–1206.
[29] S. Lulin´ ski, K. Zajac, Selective generation of lithiated benzonitriles: the impor-
tance of reaction conditions, J. Org. Chem. 73 (2008) 7785–7788.
[30] M. J. Frisch, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A., Jr. Montgomery, T. Vreven, K. N. Kudin, J. C. Burant, J. M.
Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X.
Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V.
G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.
Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,
R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M.
Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A.
Pople, Gaussian 03, Revision D.01; Gaussian, Inc., Wallingford CT, 2004.
[31] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange,
J. Chem. Phys. 98 (1993) 5648–5652.
[6] B. Strehmel, S. Amthor, J. Schelter, C. Lambert, Two-photon absorption of
bis[4-(N,N-diphenylamino)phenylethynyl]arenas, ChemPhysChem
893–896.
6 (2005)
[7] M.H.V. Werts, S. Gmouh, O. Mongin, T. Pons, M. Blanchard-Desce, Strong
modulation of two-photon excited fluorescence of quadripolar dyes by
(de)protonation, J. Am. Chem. Soc. 126 (2004) 16294–16295.
[8] F. Terenziani, A. Painelli, C. Katan, M. Charlot, M. Blanchard-Desce, Charge
instability in quadrupolar chromophores: symmetry breaking and solva-
tochromism, J. Am. Chem. Soc. 128 (2006) 15742–15755.
[9] R.D. Hreha, C.P. George, A. Haldi, B. Domercq, M. Malagoli, S. Barlow, J.-L. Brédas,
B. Kippelen, S.R. Marder, 2,7-Bis(diarylamino)-9,9-dimethylfluorenes as hole-
transport materials for organic light-emitting diodes, Adv. Funct. Mater. 13
(2003) 967–973.
[10] B.R. Cho, M.J. Piao, K.H. Son, S.H. Lee, S.J. Yoon, S.J. Jeon, M.H. Cho,
Nonlinear optical and two-photon absorption properties of 1,3,5-tricyano-
[32] R. Ditchfield, W.J. Herhe, J.A. Pople, Self-consistent molecular-orbital methods
IX. An extended Gaussian-type basis for molecular-orbital studies of organic
molecules, J. Chem. Phys. 54 (1971) 724–728.
[33] W.E. Parham, L.D. Jones, Elaboration of bromoarylnitriles, J. Org. Chem. 41
(1976) 1187–1191.
2,4,6-tris(styryl)benzene-containing octupolar oligomers, Chem. Eur. J.
(2002) 3907–3916.
8
[11] R.P. Ortiz, M.C.R. Delgado, J. Casado, V. Hernandez, O.K. Kim, H.Y. Woo, J.T.L.
Navarrete, Electronic modulation of dithienothiophene (DTT) as ␲-center of
D-␲-D chromophores on optical and redox properties: analysis by UV–vis–NIR
and Raman spectroscopies combined with electrochemistry and quantum
chemical DFT calculations, J. Am. Chem. Soc. 126 (2004) 13363–13376.
[12] W. Wang, A.D.Q. Li, Design and synthesis of efficient fluorescent dyes for incor-
poration into DNA backbone and biomolecule detection, Bioconjugate. Chem.
18 (2007) 1036–1052.
[13] E. Zojer, D. Beljonne, P. Pacher, J.L. Brédas, Two-photon absorption in quadrupo-
lar ␲-conjugated molecules: influence of the nature of the conjugated bridge
and the donor–acceptor separation, Chem. Eur. J. 10 (2004) 2668–2680.
[14] H. Sakurai, H. Sugiyama, M. Kira, Dual fluorescence of aryldisilanes and related
compounds. evidence for the formation of 1(␴␲*) orthogonal intramolecular
charge-transfer states1, J. Phys. Chem. 94 (1990) 1837–1843.
[34] J.R. Platt, Classification of spectra of cata-condensed hydrocarbons, J. Chem.
Phys. 17 (1949) 484–495.
[35] W.D. Oosterbaan, C. Koper, T.W. Braam, F.J. Hoogesteger, J.J. Piet, B.A.J.
Jansen, C.A. van Walree, H.J. van Ramesdonk, M. Goes, J.W. Verhoeven, W.
Schuddeboom, J.M. Warman, L.W. Jenneskens, Oligo(cyclohexylidene)s and
oligo(cyclohexyl)s as bridges for photoinduced intramolecular charge separa-
tion and recombination, J. Phys. Chem. A 107 (2003) 3612–3624.
[36] H. Sakurai, H. Sugiyama, M. Kira, Dual fluorescence of aryldisilanes and related
compounds: evidence for the formation of 1(␴␲*) orthogonal intramolecular
charge-transfer states, J. Phys. Chem. 94 (1990) 1837–1843.
[37] D.H. Williams, I. Fleming, Spectroscopic Methods in Organic Chemistry, 5th ed.,
McGraw-Hill Publishing Company, Berkshire, 1989.
[38] D.J. Aschaffenburg, R.S. Moog, Probing hydrogen bonding environments: sol-
vatochromic effects on the CN vibration of benzonitrile, J. Phys. Chem. B 113
(2009) 12736–12743.
[15] G. Mignani, A. Krämer, G. Puccetti, I. Ledoux, J. Zyss, G. Soula, Effect of a weak
donoronthe intramolecular chargetransfer ofmoleculescontaining two neigh-
boring silicon atoms, Organometallics 10 (1991) 3656–3659.
[39] D.R. Borst, D.W. Pratt, M. Schäfer, Molecular recognition in the gas phase.
Dipole-bound complexes of benzonitrile with water, ammonia, methanol, ace-
tonitrile, and benzonitrile itself, Phys. Chem. Chem. Phys. 9 (2007) 4563–4571.
[40] C.A. van Walree, V.E.M. Kaats-Richters, S.J. Veen, B. Wieczorek, J.H. van der
Wiel, B.C. van der Wiel, Charge-transfer interactions in 4-donor 4ꢀ-acceptor
substituted 1,1-diphenylethenes, Eur. J. Org. Chem. 10 (2004) 3046–3056.
[41] N.C. Maiti, S. Mazumdar, N. Periasamy, J- and H-aggregates of porphyrin-
surfactant complexes: time-resolved fluorescence and other spectroscopic
studies, J. Phys. Chem. B 102 (1998) 1528–1538.
[16] G. Mignani, M. Barzoukas, J. Zyss, G. Soula, F. Balegroune, D. Grandjean, D.
Josse, Improved transparency-efficiency trade-off in a new class of nonlinear
organosilicon compounds, Organometallics 10 (1991) 3660–3668.
[17] C.A. van Walree, M.R. Roset, W. Schuddeboom, L.W. Jenneskens, J.W. Verho-
even, J.M. Warman, H. Kooijman, A.L. Spek, Comparison between SiMe2 and
CMe2 spacers as ␴-bridges for photoinduced charge transfer, J. Am. Chem. Soc.
118 (1996) 8395–8407.
[18] R.M. Chen, K.M. Chien, K.T. Wong, B.Y. Jin, T.Y. Luh, Synthesis and photophysical
studies of silylene-spaced divinylarene copolymers. molecular weight depen-
dent fluorescence of alternating silylene-divinylbenzene copolymers, J. Am.
Chem. Soc. 119 (1997) 11321–11322.
[42] M. Kasha, H.R. Rawls, M. Ashraf El-Bayoumi, The exciton model in molecular
spectroscopy, Pure. Appl. Chem. 11 (1965) 371–402.