Ground and excited state intramolecular proton transfer controlled intramolecular charge separation and recombination: A new type of charge and proton transfer reaction

Article abstract of DOI:10.1016/j.chemphys.2008.02.065

A novel β-diketone 1-(4-(9-carbazol)phenyl)-3-phenyl-1,3-propanedione (CDBM) has been synthesized. When excited at 380 nm, this molecule shows single fluorescence. However, when excited at 338 nm, it shows dual fluorescence. A Al³⁺ complex Al(CDBM)₃ has been synthesized to investigate the dual fluorescence of CDBM. It is found that this complex shows single fluorescence under all excitation. This result indicated that the dual fluorescence of CDBM may relate to the intramolecular proton transfer reaction. Based on the experimental and theoretical studies of CDBM, N-(4-cyanophenyl)carbazole (CBN) and Al(CDBM)₃, a ground and excited state intramolecular proton transfer controlled intramolecular charge separation and recombination mechanism is proposed to explain the unusual excitation-dependent dual fluorescence of CDBM.

Full text of DOI:10.1016/j.chemphys.2008.02.065

Chemical Physics 348 (2008) 181–186
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Ground and excited state intramolecular proton transfer controlled
intramolecular charge separation and recombination: A new type of
charge and proton transfer reaction
b
a
a
a,
Daobo Nie a, Zuqiang Bian a, , Anchi Yu , Zhuqi Chen , Zhiwei Liu , Chunhui Huang
*
*
a Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
b Department of Chemistry, Renmin University of China, Beijing 100872, 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 b-diketone 1-(4-(9-carbazol)phenyl)-3-phenyl-1,3-propanedione (CDBM) has been synthesized.
When excited at 380 nm, this molecule shows single fluorescence. However, when excited at 338 nm,
it shows dual fluorescence. A Al3+ complex Al(CDBM)3 has been synthesized to investigate the dual fluo-
rescence of CDBM. It is found that this complex shows single fluorescence under all excitation. This result
indicated that the dual fluorescence of CDBM may relate to the intramolecular proton transfer reaction.
Based on the experimental and theoretical studies of CDBM, N-(4-cyanophenyl)carbazole (CBN) and
Al(CDBM)3, a ‘‘ground and excited state intramolecular proton transfer controlled intramolecular charge
separation and recombination” mechanism is proposed to explain the unusual excitation-dependent dual
fluorescence of CDBM.
Received 9 November 2007
Accepted 27 February 2008
Available online 18 March 2008
Keywords:
Intramolecular proton transfer
Intramolecular charge transfer
Dual fluorescence
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
1,3-propanedione (CDBM). This molecule shows single fluores-
cence (Band A), dual fluorescence (Bands A and B) as the excitation
Charge and proton transfer reactions are both fundamental
chemical processes occurring in nature. Intensive studies can be
found in the literature for both subjects [1–6]. It is particularly
interesting when both processes are feasible in one molecule.
One prototype of such systems is 40-(diethylamino)-3-hydroxyflav-
one [7,8]. This molecule exists in its energetically favorable normal
form (N) in ground state. After excitation, it is promoted to the
intramolecular charge transfer (ICT) excited state N*. At the same
time, N* can be transformed into a tautomer form (T*) through
excited state intramolecular proton transfer (ESIPT) reaction. Dual
fluorescence can be observed when the two forms decay to their
corresponding ground states [7,8]. New cases have been reported
such as HBOCE and HBODC, which show strong solvatochromic
fluorescence from ESIPT/ICT process [4]. However, the dual fluores-
cence of these molecules is excitation-independent and the charge
transfer (CT) is controlled by the intramolecular proton transfer
only in the excited states.
wavelength varies from 380 nm (around the first absorption band)
to 338 nm (around the second absorption band). Both fluorescence
bands show great solvatochromic shifts with increasing solvent
polarity. In order to study the relationship between the dual fluo-
rescence and the intramolecular hydrogen bonding of CDBM, the
complex Al(CDBM)3 has been synthesized by coordinating CDBM
to Al3+ ion. The photophysical properties (lifetimes and bathochro-
mic shifts) of the model compound N-(4-cyanophenyl)carbazole
(CBN), CDBM and Al(CDBM)3 have been studied systematically.
The excited state dipole moments of the three compounds were
determined with fluorescence solvatochromic method. Ground
state geometries and energies of these compounds were calculated
by density functional theory. Finally, the mechanism of the unu-
sual excitation-dependent dual fluorescence of CDBM was dis-
cussed based on the experimental and theoretical investigations.
2. Experimental and computational details
In this paper, we report for the first time an interesting excita-
tion-dependent dual fluorescence phenomenon of a donor–accep-
tor–acceptor type b-diketone 1-(4-(9-carbazol)phenyl)-3-phenyl-
2.1. Experimental details
N-(4-cyanophenyl)carbazole (CBN) was synthesized from
4-fluorobenzonitrile and carbazole by a simple procedure accord-
ing to the literature [9]. The crude product was purified by
* Corresponding authors. Tel./fax: +86 10 62757156.
E-mail address: chhuang@pku.edu.cn (C. Huang).
0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2008.02.065

182
D. Nie et al. / Chemical Physics 348 (2008) 181–186
Scheme 1. Synthetic procedure of CDBM.
recrystallization and subsequent column chromatography. CDBM
was synthesized as follows (Scheme 1): 2.7 g (10 mmol) CBN,
15 ml H2SO4 and 50 ml C2H5OH were added to a 100 ml round-bot-
tomed flask and heated to reflux for 3 days. The mixture was
poured into 500 ml water and then extracted with CH2Cl2. The
product ethyl 4-(9-carbazol)benzoate (4 in Scheme 1) was purified
by column chromatography with CH2Cl2–petroleum ether (1:2, v/
v) as eluent to give 2.41 g (76%) of 4. To a three-neck flask
(100 ml), sodium hydride (0.28 g, 70%, 16 mmol) washed with
anhydrous hexane and anhydrous ether (60 ml), 2.41 g (8 mmol)
4 and 1.92 g (16 mmol) acetophenone were added sequentially.
The mixture was stirred for 48 h under N2 at room temperature
and acidified by hydrochloric acid to pH 2–3, then the solvent
was vaporized and the solid was purified by silica column chroma-
tography with CH2Cl2–petroleum ether (1:1, v/v) as eluent to give
CDBM with yield of 42%. 1H NMR (CD3CN, TMS): d (ppm) 7.16 (s,
1H), 7.33 (t, 2H, J = 7.40, 7.04 Hz), 7.46 (t, 2H, J = 8.17, 7.21 Hz),
7.51–7.59 (m, 4H), 7.65 (t, 1H, J = 7.26, 7.27 Hz), 7.79 (d, 2H,
J = 8.37 Hz), 8.12 (d, 2H, J = 7.36 Hz), 8.20 (d, 2H, J = 7.76 Hz), 8.33
(d, 2H, J = 8.43 Hz), 17.14 (s, 1H). Anal. Calc. for C27H19NO2: C,
83.27; H, 4.92; N, 3.60. Found: C, 83.08; H, 4.93; N, 3.44.
tion source. By processing the lifetime data with the F900 reconvo-
lution software, a time resolution better than 1 ns for FLS920 and
20 ps for Lifespec-Red can be achieved.
2.2. Computational details
The quantum chemical calculations were performed using the
GAUSSIAN 03 package [12]. Ground state geometries of CBN, CDBM
and Al(CDBM)3 have been optimized with the B3LYP [13] func-
tional and a 6-31G(d) [14] basis set with no symmetry constraints.
Numerical frequency calculations were performed with the same
functional and basis set to determine the nature of the stationary
point. The ground state dipole moments were calculated using
the semi-empirical AM1 [15,16] method.
3. Results and discussion
3.1. Keto/enol equilibrium in solution
In general, b-diketones exist in three tautomeric forms in solu-
tion: keto, cis-enol and trans-enol form [17–19]. Scheme 2 illus-
trates the keto-enol equilibrium of CDBM. Because R1 and R2 are
different, there are more than one cis-enol form and trans-enol form
of CDBM. Scheme 2 only shows one of them for clarity. Among these
tautomers, cis-enol forms are considered to be the photochemical
stable ones because they are stabilized by the intramolecular hydro-
gen bonding and the conjugated system [17,20]. Theoretical studies
have shown that the energies of trans-enol form b-diketones are
much higher than their cis-enol isomers [20,21]. Therefore, they
can only be observed experimentally at very low temperatures
[21,22]. Because our experiments were all carried out at room tem-
perature, it is reasonable to only consider the existence of keto and
cis-enol forms. 1H NMR spectrum of ca. 10ꢀ3 mol lꢀ1 CDBM in
The complex Al(CDBM)3 was synthesized according to the fol-
lowing procedure: To
a 50 ml round-bottomed flask, CDBM
(1.167 g, 3.0 mmol), NaOH (0.120 g, 3.0 mmol) were mixed in
10 ml ethanol and refluxed for 10 min, then it was added dropwise
to 10 ml ethanol solution of AlCl3 (0.146 g, 1.1 mmol). The mixture
was refluxed for 2 h and then poured into water. The crude product
was obtained by filtration and purified by recrystallization in a
mixture of THF–ethanol. 1H NMR (CDCl3, TMS): d (ppm) 7.11 (t,
3H, J = 8.03, 7.56 Hz), 7.29 (d, 6H, J = 7.50 Hz), 7.38–7.49 (m,
21H), 7.63–7.67 (m, 6H), 8.13 (t, 12H, J = 8.12, 7.14 Hz), 8.31–
8.34 (m, 6H). Anal. Calc. for C81H54AlN3O6: C, 81.60; H, 4.57; N,
3.52. Found: C, 80.95; H, 4.72; N, 3.82.
All spectroscopic grade solvents were purchased from ACROS.
Absorption spectra were recorded with Shimadzu UV 3100
spectrophotometer, and steady state fluorescence spectra were ta-
ken with an Edinburgh Instruments FLS920 spectrometer. All the
fluorescence spectra were corrected for the instrument response
using a correction file provided by the manufactory. Fluorescence
quantum yields were determined using quinine sulfate in 1.0 N
H2SO4 as a standard (Uf = 0.546 at 25 °C) [10,11].
Lifetimes longer than 1 ns were measured with Edinburgh
Instruments FLS920 based on the time correlated single photon
counting technology. A hydrogen filled nanosecond flash lamp
was used as the excitation source and the signals were detected
with a photomultiplier (Hamamatsu R955). A picosecond lifetime
spectrometer Edinburgh Instruments Lifespec-Red was used to
measure lifetimes shorter than 1 ns. In this equipment, a diode la-
ser (operating at 372 nm, pulse duration 69 ps) controlled by a
picosecond light pulser (Hamamatsu PLP-10) was used as excita-
Scheme 2. Keto-enol equilibrium of CDBM.

D. Nie et al. / Chemical Physics 348 (2008) 181–186
183
CD3CN indicated that this molecule exists in solution predomi-
nantly as the cis-enol form (NMR signals at 17.0 ppm of OH and at
7.0 ppm of acetyl proton, no signal at 3.5–4.5 ppm corresponding
to a-diketonic protons [23]). Based on the fact that the percentage
of cis-enol forms will increase as the solvent polarity decreases
[17], and that CH3CN is the strongest polar solvent in our experi-
ments, it is reasonable to suppose that CDBM exists mainly as the
cis-enol forms in our experimental conditions. There are actually
two possible structures of cis-enol form CDBM as shown in Scheme
3. In cis-enol-A, the intramolecular hydrogen bonding is O1–
H8ꢁ ꢁ ꢁO5 while in cis-enol-B, it is O5–H8ꢁ ꢁ ꢁO1. These two cis-enol
form isomers can transform into each other through a transition
state (TS) [20]. Because the chemical environments of these two iso-
mers are very similar to each other, it is possible that they are not
distinguishable in the 1H NMR spectrum. We calculated the relative
energies of the two tautomers and the transition state (TS) with
B3LYP/6-31G(d) method. The results showed that cis-enol-A (re-
laxed form) lies only 0.6 kJ molꢀ1 above cis-enol-B (relaxed form)
and the energy barrier between the two tautomers is about
9 kJ molꢀ1. This energy difference between the two tautomers is
so small that they should be able to exist in solution simultaneously.
3.2. Ground state structures
The B3LYP/6-31G(d) optimized 3D structures of the two cis-enol
forms CDBM and Al(CDBM)3 are illustrated in Fig. 1. The bond
lengths of the central b-diketone fragment are also shown. As
shown in Fig. 1, the DBM part (Chart 1) is nearly coplanar in all
the three molecules. The carbazole moiety is twisted by about
50° with respect to phenyl ring I (Chart 1). The intramolecular
hydrogen bonding of cis-enol-A is O1–H8ꢁ ꢁ ꢁO5 and that of cis-
enol-B is O5–H8ꢁ ꢁ ꢁO1. Both intramolecular hydrogen bonds exhibit
a bond length of 1.58 Å, which indicates strong hydrogen bonding
in both isomers. In cis-enol-A form CDBM, C2–O1 is close to a sin-
gle bond and C4–O5 is close to a double bond. However, the situ-
ation is totally opposite in cis-enol-B isomer. From this result we
can infer that the Donor–p–Acceptor (D–p–A) system of cis-enol-
B extends from the carbazole moiety through phenyl ring I to the
carbonyl group C2–O1, while that of cis-enol-A spreads over the
carbazole moiety, phenyl ring I, C2–C3, and carbonyl group C4–
O5, which is obviously larger. Significant changes have taken place
in the b-diketone part of CDBM after coordinating with Al3+. The
intramolecular hydrogen bond has been eliminated and all the
bond lengths in the b-diketone part (C–O bonds and C–C bonds)
have been equalized. As a result, the b-diketone part loses its quin-
oid structure and exhibits a conjugated one. The whole b-diketone
part can be regarded as the acceptor. Therefore, the D–p–A system
of Al(CDBM)3 should be extended from carbazole moiety through
phenul ring I to the center of b-diketone. Based on these facts,
we suppose that the D–p–A system of Al(CDBM)3 should be inter-
mediate between those of cis-enol-A and cis-enol-B isomers.
Fig. 1. The optimized structures of cis-enol-A (top), cis-enol-B (middle) and Al(C-
DBM)3 (bottom). The bond lengths are in angstroms (Å). Hydrogen atoms in Al(C-
DBM)3 are omitted for clarity.
3.3. Absorption
The absorption spectra of CBN, DBM, CDBM and Al(CDBM)3 in
CH2Cl2 are shown in Fig. 2. By comparing the absorption spectrum
Chart 1. Molecular structure of CDBM, CBN and DBM.
of CDBM with that of DBM, it is reasonable to argue that the lon-
gest-wavelength absorption band (Ex 1 in Fig. 2, ꢂ380 nm) and
the shortest-wavelength band (ꢂ290 nm) is related to the carba-
zole moiety, which is the only difference between CDBM and
DBM in molecular structure (Chart 1). The absorption spectra of
CBN and CDBM are very similar from 260 nm to 338 nm. They both
have a strong and structured absorption band around 290 nm. As
Scheme 3. The equilibrium between the two cis-enol form CDBM. The structures of
R1 and R2 are shown in Scheme 2.

184
D. Nie et al. / Chemical Physics 348 (2008) 181–186
Fig. 2. Absorption spectra of CBN (dash), DBM (thin solid), CDBM (thick solid) and
Al(CDBM)3 (dot) in CH2Cl2.
carbazole showed a strong, sharp absorption band at 291.7 nm
[24], we assigned this absorption band to the locally excited (LE)
state (p* p transition) of the carbazole moiety. Around 338 nm,
CDBM and CBN both show a sharp absorption band (Ex 2). Rettig
and Zander assigned this band of CBN to a CT state which is popu-
lated by an electron transfer from carbazol moiety to the cyano-
phenyl part [25]. However, in the region above 350 nm, the
absorption spectrum of CBN suddenly decreased to zero but CDBM
showed a broad and strong absorption band around 380 nm. From
Section 3.1 we know that there are two possible tautomers of
CDBM in solution. Therefore, it is reasonable to suppose that the
two absorption bands of CDBM (around 338 nm and 380 nm,
respectively) may be related to the two tautormers (cis-enol-A
and cis-enol-B).
Fig. 3. Corrected fluorescence spectra of 1 ꢃ 10ꢀ5 M CDBM in various solvents. I:
excited at 380 nm; II: excited at 330 nm. The numbers refer to solvents in Table 1.
The arrows indicate the positions of Band B and the triangles denote the peaks of
Band A. Please notice that arrows and triangles point to traces in the same color.
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
3.4. Emission and excitation
CDBM exhibits interesting excitation-dependent dual fluores-
cence in solution. If the excitation varies from 350 nm to
ꢂ400 nm, CDBM will show single fluorescence (referred as ‘‘Band
A”) in all solvents (Fig. 3, I). The spectrum shows vibrational struc-
ture in cyclohexane, suggesting partially LE character of the excited
state [19]. In other solvents, the emission spectra show a broad
structureless band which is strongly red-shifted as the solvent
polarity increases. As the most characteristic phenomenon of
charge transfer excited state is the bathochromic shift with
increasing solvent polarity, this result indicates the CT character
of Band A. However, if the excitation wavelength is shorter than
350 nm, CDBM will exhibit dual fluorescence in all solvents
(Fig. 3, II). In addition to Band A, a new band (Band B) emerges
on the short-wavelength side. This new band also shows a batho-
chromic shift with increasing solvent polarity, but not as promi-
nent as Band A. In low polar solvents, Band B prevails. The
intensity ratio of (Band B)/(Band A) gradually decreases as the sol-
vent polarity increases. In strong polar solvents, Band A becomes
dominate.
Because the fluorescence spectra depend on the excitation
wavelength, it is important to investigate the excitation spectra
of both emission bands. Fig. 4 shows the excitation spectra of
CDBM monitored at both fluorescence bands together with the
absorption spectra of CDBM and CBN in THF. As can be seen from
Fig. 4, the excitation spectra monitored at both emission bands
are quite different from each other, indicating different species in
ground state. The one monitored at Band A is very similar to its
absorption spectrum. However, the excitation spectrum monitored
at Band B is quite different: in the region above 338 nm, it falls
steeply down to zero. Interestingly, it closely resembles the
Fig. 4. Top panel: excitation spectra of 1 ꢃ 10ꢀ5 M CDBM in THF monitored at di-
fferent emission bands. Bottom panel: UV–Vis spectra of 1 ꢃ 10ꢀ5 M CDBM and CBN
in THF.
absorption spectrum of CBN. This result indicates that the molecu-
lar structure of the species corresponding to Band B may be similar
to CBN.
As discussed in Section 3.1, cis-enol-A and cis-enol-B may both
exist in solution. The excitation spectra monitored at both emis-
sion bands also indicate different species in ground state. There-
fore, the excitation-dependent dual fluorescence may come from
the two cis-enol forms of CDBM. To verify this assumption, we
studied the fluorescence spectra of Al(CDBM)3, in which the intra-
molecular hydrogen bonding of CDBM has been eliminated by
coordinating with Al3+. The fluorescence spectra of Al(CDBM)3 in
THF under different excitation are shown in Fig. 5. The dual fluo-
rescence spectrum of CDBM in THF is also depicted for comparison.

D. Nie et al. / Chemical Physics 348 (2008) 181–186
185
ute in the ground and excited states, respectively, e0 is the absolute
permittivity of vacuum, a0 is the effective radius of the Onsager cav-
ity[26], e is the static dielectric constant, and n is the refractive in-
CT
flu
dex of the solvent. By plotting ~t against the solvent polarity
parameter f(e, n) = [f(e) ꢀ 0.5f(n)], where
e ꢀ 1
n2 ꢀ 1
fðeÞ ¼
;
fðnÞ ¼
ð2Þ
2e þ 1
2n2 þ 1
*
*
we can get the values of 2*leðle ꢀ lgÞ=4pe0a30. To determine the ex-
cited state dipole moment, we need to know the ground state dipole
moment lg, the radius of the Onsager cavity a0 and the angle be-
tween le and lg. We performed AM1 calculations on the B3LYP/6-
31G(d) optimized structures of CBN, cis-enol-A, cis-enol-B CDBM
and Al(CDBM)3 and got ground state dipole moments 2.52 D, 2.54
D, 3.00 D, 2.09 D for the four structures, respectively. As all the
ground state dipole moments are very small, we ignored the angle
between le and lg. The estimation of a0 is very important to deter-
mine le as it enters the equation to the third power. Different
authors have taken different approaches to calculate the value of
a0 [32–34]. We used the procedure recommended by Karpiuk in a
recent paper [32]. For CBN, we estimated a0 to be half the average
distance of C6–N1 (see Chart 1 for atom numbering) and C7–N1
and got a0 = 3.86 Å. For cis-enol-B form CDBM (Band B), half the
average distance of C6–O1 and C7–O1 was taken and we got
a0 = 3.64 Å. For cis-enol-A form CDBM (Band A), we took half the
average distance of C6–O5 and C7–O5 and yielded a0 = 4.70 Å. In
Al(CDBM)3, the two oxygen atoms (O1 and O5) both act as the
acceptor groups, so we took the average a0 of the two cis-enol forms
and got 4.17 Å. With all these results and the data in Table 1, we ob-
tained the excited state dipole moments and the results are listed in
Table 2. Samanta employed a ‘‘time-resolved microwave dielectric
loss” method to determine the excited state dipole moment of
CBN to be 10.4 D [35], which is very close to our result (10.1 D). This
agreement proved our method to be reliable. It can be seen from Ta-
ble 2 that the excited state dipole moment of Band B is very close to
that of CBN. However, Band A shows a much larger excited state di-
pole moment. From Section 3.2 we know that the D–p–A system of
Fig. 5. The fluorescence spectra of Al(CDBM)3 and CDBM in THF. Thick solid blue
line: Al(CDBM)3 excited at 330 nm; dashed red line: Al(CDBM)3 excited at 380 nm;
thin solid black line: CDBM excited at 330 nm. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this
article.)
As expected, Al(CDBM)3 showed single fluorescence independent
of the excitation wavelength. This result strongly suggests that
the dual fluorescence of CDBM is caused by the intramolecular pro-
ton transfer reaction, which results in the equilibrium between cis-
enol-A and cis-enol-B.
However, the corresponding relationship between the two
emission bands and the two tautomers is still unclear. In order to
solve this problem, we studied the photophysical properties (band
positions and lifetimes) of CBN, cis-enol-A, cis-enol-B and
Al(CDBM)3 in various solvents. The results are listed in Table 1. It
can be noticed from Table 1 that all the emission bands showed
strong bathochromic shifts with increasing solvent polarity, indi-
cating charge transfer nature of their corresponding excited states.
The lifetimes of the four species are all in a nanosecond range (ex-
cept for Band A in cyclohexane) which is two or three orders of
magnitude longer than the solvent orientational times. Based on
this fact, the relaxed excited state dipole moments can be esti-
mated employing the modified Lippert–Mataga equation (1)
[17,26–31]:
Table 2
Ground and excited state dipole moments of CBN, cis-enol-B, cis-enol-A and
Al(CDBM)3
ꢀ
ꢁ
*
*
2*leðle ꢀ lgÞ e ꢀ 1
1 n2 ꢀ 1
Fluorescence slope (cmꢀ1
)
le (D)
CT
vac
flu
Molecule
lg (D)
a0
hc~t ffi hc~t
ꢀ
ꢀ
ð1Þ
flu
4pe0a30
2e þ 1 2 2n2 þ 1
Al(CDBM)3
Band A
Band B
CBN
2.09
2.54
3.00
2.52
4.17
4.70
3.64
3.86
ꢀ18111
ꢀ20246
ꢀ14923
ꢀ13670
12.4
15.7
10.0
10.1
CT
where h is the Planck constant, c is the speed of light in vacuum, ~t
flu
vac
and ~t are the CT fluorescence band positions in solution and in
flu
vacuum, respectively, *lg and le are the dipole moments of the sol-
*
Table 1
CT emission band positions (cmꢀ1, nm in the parentheses) and lifetimes (ns) for Al(CDBM)3, CDBM and CBN in various solvents
Solvent
f(e, n)a
Al(CDBM)3
Peak
Band A
Peak
Band B
Peak
CBN
s (ns)
s (ns)
s (ns)
Peak
s (ns)
Acetonitrile (1)
Acetone (2)
1,2-Dichloroethane (3)
Dichloromethane (4)
Methyl acetate (5)
THF (6)
Ethyl acetate (7)
n-Butyl acetate (8)
Trichloromethane (9)
Aether (10)
0.393
0.374
0.326
0.319
0.308
0.307
0.292
0.266
0.256
0.253
0.126
0.100
18,250 (548)
19,720 (507)
19,920 (502)
20,330 (492)
20,620 (485)
21,690 (461)
21,500 (465)
22,170 (451)
21,930 (456)
23,040 (434)
23,260 (430)
24,390 (410)
4.8
4.1
4.7
3.6
2.2
1.0
1.2
0.6
0.9
0.2
0.4
0.1
17,890 (559)
19,190 (521)
19,650 (509)
19,650 (509)
19,530 (512)
21,230 (471)
21,370 (468)
21,830 (458)
20,700 (483)
22,830 (438)
23,260 (430)
24,810 (403)b
3.9
5.2
5.8
5.5
3.2
2.0
2.1
1.2
3.2
0.2
22,990 (435)
23,700 (422)
24,210 (413)
23,980 (417)
24,690 (405)
25,130 (398)
25,060 (399)
25,320 (395)
24,330 (411)
25,770 (388)
26,530 (377)
28,990 (345)
7.9
6.7
8.3
7.9
7.0
6.0
5.6
5.7
6.5
4.1
4.1
3.6
23,810 (420)
24,810 (403)
24,940 (401)
24,940 (401)
25,060 (399)
25,450 (393)
25,450 (393)
25,710 (389)
25,190 (397)
26,530 (377)
26,740 (374)
28,990 (345)
7.5
6.6
7.6
7.4
6.3
5.8
5.2
5.0
5.9
3.8
4.0
3.5
Toluene (11)
Cyclohexane (12)
0.1
<0.02
a
Solvent polarity parameter, see Eq. (2) in the following text.
The emission band position corresponding to 0–0 transition.
b

186
D. Nie et al. / Chemical Physics 348 (2008) 181–186
experimental and theoretical studies, it is concluded that the unu-
sual excitation-dependent dual fluorescence originates from the
charge transfer reaction which is controlled by the intramolecular
proton transfer reaction in both ground and excited states.
Acknowledgement
The authors thank the National Basic Research Program
(G2006CB601103), National Nature Science Foundation of China
(20471004, 20490210, 20021101, 90401028) for financial support.
Appendix A. Supplementary data
Cartesian coordinates for the optimized geometries of cis-enol-
A, cis-enol-B form CDBM and Al(CDBM)3. Supplementary data asso-
ciated with this article can be found, in the online version, at
doi:10.1016/j.chemphys.2008.02.065.
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3.5. Excitation-dependent dual fluorescence of CDBM
Based on the results and discussions presented above, a possible
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Band A will become dominant.
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4. Conclusion
We report a novel synthesized b-diketone CDBM which displays
a new type of charge and proton transfer reaction. Based on the