Thermodynamics and conformations in the formation of excited states and their interconversions for twisted donor-substituted tridurylboranes

Article abstract of DOI:10.1002/chem.201201719

We synthesized a series of donor-substituted tridurylboranes containing different types and number of chromophores including 1-pyrene (PB1-3), 3-carbazole (CBC1-3), or substituted p-carbazol-N-phenyl (CBN3a-c) as various donor-acceptor (D-A) molecules. The photophysical and electrochemical properties of these twisted D-A molecules were investigated by means of UV/Vis absorption and fluorescence spectroscopy as well as cyclic voltammetry (CV). Solvent polarity, viscosity, and temperature effects on the fluorescence emission reveal the existence of three types of excited states, and their equilibria and interconversions between three excited states. In increasing order of the charge-separated extent and the conformational change, three excited states are the locally excited (LE) state, the more planar intramolecular charge-transfer (ICT) state, and the more twisted ICT (TICT) state as compared to the ground state. The TICT state undergoes a conformational change with a higher energy barrier over the ICT state. The solvent polarity effect on the state conversion is opposite to the viscosity effect, and temperature effects derive from its resulting changes of polarity and viscosity. For example, the increase of the polarity of the solvent results in excited-state conversions from the LE state to the ICT state, and/or from the ICT to the TICT state, and an increased viscosity leads to the opposite conversions. On the basis of electrochemical and spectral data, thermodynamics of a possible ICT process were estimated, and correlated with the excited-state character. Finally, three excited states have been characterized by the conformation, the photophysical properties, and the thermodynamics of the ICT processes. Directed interconversion: Three types of excited states (LE, ICT, and TICT) in the photochemical processes of a series of synthesized tridurylboranes are characterized by the conformation, the photophysical properties, and the thermodynamics of the intramolecular charge-transfer process. Effects of the solvent polarity, the viscosity, and the temperature on the fluorescence spectra reveal the interconversion and equilibria between the excited states (see figure). Copyright

Full text of DOI:10.1002/chem.201201719

DOI: 10.1002/chem.201201719
Thermodynamics and Conformations in the Formation of Excited States and
Their Interconversions for Twisted Donor-Substituted Tridurylboranes
Mao Mao, Ming-Guang Ren, and Qin-Hua Song*[a]
Abstract: We synthesized a series of
donor-substituted tridurylboranes con-
taining different types and number of
states. In increasing order of the
charge-separated extent and the con-
formational change, three excited
states are the locally excited (LE)
state, the more planar intramolecular
charge-transfer (ICT) state, and the
more twisted ICT (TICT) state as com-
pared to the ground state. The TICT
site to the viscosity effect, and temper-
ature effects derive from its resulting
changes of polarity and viscosity. For
example, the increase of the polarity of
the solvent results in excited-state con-
versions from the LE state to the ICT
state, and/or from the ICT to the TICT
state, and an increased viscosity leads
to the opposite conversions. On the
basis of electrochemical and spectral
data, thermodynamics of a possible
ICT process were estimated, and corre-
lated with the excited-state character.
Finally, three excited states have been
characterized by the conformation, the
photophysical properties, and the ther-
modynamics of the ICT processes.
chromophores
(PB1–3), 3-carbazole (CBC1–3), or
substituted p-carbazol-N-phenyl
including
1-pyrene
(CBN3a–c) as various donor–acceptor
(D–A) molecules. The photophysical
and electrochemical properties of these
twisted D–A molecules were investi-
gated by means of UV/Vis absorption
and fluorescence spectroscopy as well
as cyclic voltammetry (CV). Solvent
polarity, viscosity, and temperature ef-
fects on the fluorescence emission
reveal the existence of three types of
excited states, and their equilibria and
interconversions between three excited
state undergoes
a
conformational
change with a higher energy barrier
over the ICT state. The solvent polarity
effect on the state conversion is oppo-
Keywords: boranes · charge trans-
fer · conformational analysis · do-
nor–acceptor systems · thermody-
namics
Introduction
ear optical and charge-transport materials, emitters in or-
ganic light-emitting devices (OLEDs), selective and sensi-
tive chemosensors for the detection of fluoride and cyanide
ions,[1,2d,e,g,h,i] and a fluorescent thermometer.[3] However,
studies on the structure of the excited states of triarylbor-
anes are scarce.[2c,4]
In the field of ICT between strongly coupled organic D
and A fragments, the molecular structure (including elec-
tronic structure and molecular geometry) in the ICT state
has been under controversial discussion,[5] even for the
Photoinduced intramolecular charge-transfer reactions are
some of the most frequently encountered photochemical pri-
mary processes. In such intramolecular processes for a con-
jugated system with donor (D) and acceptor (A) subunits,
the reaction product is usually called an intramolecular
charge-transfer (ICT) state, which often is the excited state
with a somewhat increased dipole moment for a D–A mole-
cule. Owing to the vacant p orbital on the boron atom
(p(B)), the excited states of triarylboron compounds possess
more or less the character of an ICT from an aryl group to
the p(B). The ICT interaction plays essential roles in deter-
mining electronic structures of triarylboron compounds, and
thereby, its photophysical and photochemical properties.[1,2]
The intriguing electronic and photophysical properties of tri-
arylboron derivatives result in many applications for nonlin-
well-studied
molecule,
N,N-dimethylaminobenzonitrile
(DMABN). Since the first observation by Lippert et al.,[6]
the phenomenon of dual fluorescence for DMABN has led
to numerous theoretical and experimental studies to account
for the origin of the ICT fluorescence.[5,6–12] Among them,
two important hypotheses are 1) a twisting of the dimethyl-
AHCTUNGTREGaNNNU mino (D)–benzonitrilo (A) single bond that results in a
nearly perpendicular D–A geometry (twisted intramolecular
charge transfer, TICT),[5,7,8] 2) a planarization of the amino
group (planar intramolecular charge transfer, PICT).[9–12] Al-
though the TICT model has gained widespread acceptance,
the reaction mechanism is still under discussion.[5f,7b,7d,13] Es-
pecially, the twisted angle necessary for the formation of the
TICT state in derivatives of DMABN is not quite clear. For
example, a torsional barrier is present in the conventional
adiabatic precursor (the locally excited (LE) state)–succes-
sor (TICT) model, but it has been shown that the TICT
[a] Dr. M. Mao, Dr. M.-G. Ren, Prof. Dr. Q.-H. Song
Department of Chemistry
Joint Laboratory of Green Synthetic Chemistry
University of Science and Technology of China
Hefei 230026, Anhui (P. R. China)
Fax: (+86)551-3601592
E-mail: qhsong@ustc.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201719.
15512
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15512 – 15522

FULL PAPER
state can be formed directly from the Franck–Condon state
on S2 through a conical intersection corresponding to a non-
adiabatic and nearly barrierless reaction route.[14] Recently,
another theoretical study on this issue has revealed that
dual fluorescence of D- and A-substituted benzene deriva-
tives including DMABN could result from the LE-PICT as
well as the LE-TICT states, depending on the molecular
structures.[15]
Despite numerous studies on the ICT behavior of conju-
gated D–A systems, predicting their character of the excited
states simply based on their molecular structure remains a
challenging task. Thermodynamic expectations to the stabili-
ty of the TICT state of biaryls were performed based on the
reversible (oxidation and reduction) half-wave potentials
and the energies of the primary excited state, or the gas-
phase ionization potential (IP) and electron affinity (EA)
values, by Zachariasse,[9a,16] as well as Zander and Rettig.[17]
The thermodynamic estimates agree in indicating a special
stability of the CT states of 9,9’-bianthracene, 3,3’-bipery-
lene, or 1,1’-bipyrene and a thermodynamic instability of the
TICT states of several other biaryls versus their nonpolar
excitonic states. However, the thermodynamic estimates are
not decisive for the stability of the TICT state for most bi-
stituents at the duryl groups. The tridurylboranes are com-
posed of 1) different donors: 1-pyrene (PB1–3), 3-carbazole
(CBC1–3a) its 6-methoxy-substituted derivative (CBC3b),
p-carbazol-N-phenyl (CBN3a), and its 3,6-substituted deriv-
atives (tBu: CBN3b, CH3O: CBN3c); 2) different acceptors:
tri- (PB1 and CBC1), di- (PB2 and CBC2), and mono-
(PB3, CBC3a,b, and CBN3a–c) donor substitution. By utiliz-
ing these compounds, we have investigated the influence of
the solvent polarity, the viscosity, and the temperature on
their fluorescence spectra. On the basis of photophysical
and electrochemical studies, we have gained some new in-
sights into the conformational and photophysical behavior,
and thermodynamics of ICT process in these D–A mole-
cules.
Results and Discussion
Synthesis of tridurylboranes: The synthetic procedures for
the tridurylboranes are shown in Scheme 1. 1,4-Dibromo-
2,3,5,6-tetramethylbenzene (1)[19] and tri-p-bromodurylbor-
ane (2)[19] were prepared with 1,2,4,5- tetramethylbenzene as
starting material. The reactions of 2 with different amounts
of CuCN were carried out at 1408C in DMF solutions to
give the cyano-substituted compounds[18] 3–5, respectively.
The palladium-catalyzed Suzuki coupling reaction of the p-
bromodurylboron derivatives 2–4 with 4.5, 2.5, or 1.2 equiva-
lents of the aryl-substituted boronic acids (6–8 shown in
Scheme 2) afforded the corresponding tridurylboranes, PB1–
3,[18] CBC1–3, and CBN3a–c, respectively. Among these
aryl-substituted boronic acids, compounds 6,[20] 7a,[21] 8a,[22]
and 8b,[23] were prepared in terms of reported methods. Syn-
thetic procedures of compounds 7b and 8c are provided in
the Supporting Information.
ACHTUNGTRENNUNG
bria and interconversions between three excited states of
pyrene-substituted tridurylboranes PB1–3, a LE state, and
two CT states that involve conformational changes. In these
donor-substituted tridurylboranes, D–A units can be clearly
defined and they would therefore serve as excellent model
compounds for the study of photoinduced charge transfer.
To further understand the excited-state properties of tri-
durylboranes, we have synthesized a series of carbazole
(Cz)-substituted tridurylboranes by varying the para sub-
Scheme 2. Chemical structures of the aryl-substituted boronic acids 6–8.
Scheme 1. Synthetic procedures for the tridurylboranes: a) Br2, I2, CH2Cl2; b) i) nBuLi (1 mol), Et2O, ꢀ788C; ii) BF3·OEt2 (0.3 mol), ꢀ788C to RT;
c) CuCN, DMF, 1408C; d) the corresponding aryl boronic acid, and 5–17 mol% [Pd(PPh3)4], toluene, EtOH, K2CO3, heating to reflux.
AHCTUNGTRENNUNG
Chem. Eur. J. 2012, 18, 15512 – 15522
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15513

Q.-H. Song et al.
Structures and spectral properties of the tridurylboranes:
Among these synthesized tridurylboranes, the structure of 5
has been determined by X-ray crystallography, as shown in
Figure 1. The molecule has a crystallographic C2 axis along
pyrene-substituted tridurylboranes, there is a slightly strong
conjugation between the duryls and the carbazole(s) in car-
bazole-substituted tridurylboranes (CBC1–3 and CBN3a–c)
by comparing free carbazoles, N-methylcarbazole, or the
substituted carbazoles. The UV absorption and emission
spectra, and spectral data of related compounds are provid-
ed in Figure S2 and Table S1 in the Supporting Information.
Although the UV absorption spectra are similar for the
same group of tridurylboranes, the maximal absorption
bands broaden from PB1 to PB3, and from CBC1 to CBC3,
and slightly redshifts from CBN3a to CBN3c. With decreas-
ing the number of pyrene or carbazole units for PB1–3 and
CBC1–3, the corresponding molar extinction coefficient de-
creases.
In contrast to the UV absorption spectra, the fluorescence
spectra of these compounds in THF display a large differ-
ence in the fluorescence maxima. Except the fluorescence
spectrum of PB1 is the structured emission like that of
pyrene, which is assigned to the locally excited state, other
tridurylboranes emit single-band structureless fluorescence,
which are assigned to charge-transfer emissions. Their fluo-
rescence peaks redshift from PB1 to PB3, from CBC1 to
CBC3, and from CBN3a to CBN3c. Hence, their Stokes
shifts increase gradually in orders of PB1–3, CBC1–3, and
CBN3a–c, respectively, for the data see Table S2 in the Sup-
porting Information.
Figure 1. X-ray crystal structure of 5. ORTEP plot of the molecular struc-
ture. Displacement ellipsoids are shown at the 50% probability level.
ꢀ
with the B CPh bond. The central boron is completely trigo-
nal planar, and three duryl groups are arranged in a propel-
ler-like fashion in which the dihedral angles between each
duryl plane and the boron plane are about 598. This struc-
ture is similar to the crystal structure of a tridurylboron de-
rivative reported by Yamaguchi et al.[19]
The spectral properties of the tridurylboranes in THF sol-
utions are shown in Figure 2. Compared to that of pyrene,
both absorption spectra of PB1–3 reveal a small redshift,
and reflect a weak conjugation between the pyrene and the
duryl group due to their steric hindrance. Relative to
The excited-state character under different conditions
Solvatochromic effects: The fluorescence spectra of the tri-
durylboranes in various polarity solvents are showed in
Figure 3. These fluorescence spectra show large solvatochro-
mic shifts, and these shifts increase from CBC1 to CBC3a,b
and from CBN3a to CBN3c. CBC3b and CBN3c are non-
fluorescent in three strongly polar solvents and CBN3b is
non-fluorescent in acetonitrile. This indicates a large differ-
ence in the dipole moments between the excited states and
their parent ground states. The dipole moment (me) of the
fluorescent state can be estimated from the slope (mf) of the
plot of the energies of the fluorescence maxima (nf) against
the solvent parameters Df (Figure S3, see the Supporting In-
formation) in terms of Equations (1)–(3),[24] in which mg is
the ground-state dipole moment, a is the solvent cavity
(Onsager) radius, which was derived from the number of
Avogadro (N), the molecular weight (M), and the density
(d=1), and e, e0, and n are the solvent dielectric constant,
the vacuum permittivity, and the solvent refractive index, re-
spectively.
2meðme ꢀ mgÞ
ð1Þ
ð2Þ
ð3Þ
nf ¼ ꢀ
Df ¼
Df þ C
4pe0hca3
e ꢀ 1 0:5ðn2 ꢀ 1Þ
ꢀ
2e þ 1
2n2 þ 1
1
3
3M
4Npd
Figure 2. UV absorption (solid lines) and fluorescence (dash lines) spec-
tra of 0.01 mm solutions of the tridurylboranes in THF.
a ¼ ð
Þ
15514
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15512 – 15522

Twisted Donor-Substituted Tridurylboranes
FULL PAPER
electron-donating ability of the donor, the larger dipole mo-
ments for single-donor systems CBC3a,b and CBN3a–c.
However, the values of me render irregular changes. If a cal-
culated center-to-center distance (r) between two charges is
employed to replace the cavity radius (a), new excited-state
dipole moments obtained from Equation (1) have a similar
trend to the mg values. These values of r and the correspond-
ing values of me are listed in brackets in Table 1.
The increased dipole moments of the tridurylboranes re-
flect an increase in the charge-separation extents. If it is a
full charge separation, the dipole moments should be
around 39 D (=4.8 D/ꢁꢂ8.2 ꢁ) for the PB series, about
48 D (=4.8 D/ꢁꢂ10 ꢁ) for the carbazole-substituted com-
pounds. Thus, the charge-separation extents are the lowest
60% for PB2 and the highest 95.7% for CBC3b. Large
dipole moments (18.8–26.9 D or 23.6–46.8 D) and high
charge-separation extents are evidence for the formation of
a TICT state.[4,5f,25] Although most tridurylboranes display
CT emission to different extents, the fluorescence spectra of
PB1 exhibit LE emission in most solvents. Hence, there may
be three types of excited states: the LE state, the TICT
state, and an intermediate ICT state, in photochemical pro-
AHCTUNGTRENGNcUN esses of these tridurylboranes.
In rigid glasses at 77 K: To gain further excited-state proper-
ties of the tridurylboranes, fluorescence spectra of the tri-
durylboranes in two optically transparent rigid glasses,
methyltetrahydrofuran (MTHF) or ethanol, at 77 K, were
recorded and are shown in Figure 4. All spectra of PB1–3 in
the two solvents reveal the characteristic emission peaks
(l=378, 397 nm) similar to pyrene, which are assigned to
the LE states. Compared to those of PB1 at room tempera-
ture,[18] the structured fluorescence with the two peaks is in-
dependent of the solvent and the temperature. In the case
of the CBC series, only CBC1 emits a characteristic fluores-
cence with two peaks (l=359, 379 nm) in the two glassy-
state solutions, and CBC2 and CBC3a have a weak peak at
l=359 nm. The characteristic fluorescence is also indepen-
Figure 3. Fluorescence spectra of CBC1–3 and CBN3a–c in various sol-
vents: 1) n-hexane, 2) toluene, 3) dioxane, 4) chloroform, 5) THF,
6) CH2Cl2, 7) 1,2-dichloroethanol (DCE), 8) ethanol, 9) acetone, and
10) acetonitrile.
The value of mg was calculated at the B3LYP-31G level,
and the values of a, mg, mf, and me are listed in Table 1.
The ground-state dipole moments (mg) increase from PB1
to PB3, from CBC1 to CBC3b, and from CBN3a to CBN3c.
Obviously, the single-donor systems reveal the largest dipole
moments among the PB or CBC series, and the stronger
AHCTUNGTERGdNNUN ence of the solvent, and as-
signed to the LE emission.
CBN3a–c display single-band
emissions and the fluorescence
maxima undergo redshifts. In
the polar solvent ethanol, the
long-band emission intensity in-
creases for PB1–3 and CBC1,
and their emission bands red-
shift for CBC2, CBC3, and
CBN3a–c, relative to those in
the lowly polar solvent MTHF.
For the tridurylboranes with
Table 1. Dipole moments of the ground states (mg) and the excited states (me), redox potentials, and free
energy changes (DG0) for a TICT process of the tridurylboranes.
[e]
[f]
[g]
Compd mg[a] [D] a(r)[b] [ꢁ] mf[c] [cmꢀ1
]
me[d] [D]
Eox [V] ꢀEred[e] [V] lem [nm]
DG0 [eV]
PB1
PB2
PB3
CBC1
CBC2
CBC3a 7.92
CBC3b 9.20
CBN3a 3.76
CBN3b 4.53
CBN3c 4.94
0.125
5.37
5.66
2.04
4.82
7.37 (8.2)
6.92 (8.2)
–
7917
–
1.36
2.27
2.05
1.83
2.35
2.10
1.85
1.90
1.87
1.89
1.91
377, 397
377, 397 (77 K)
0.34
0.17
18.8 (23.6) 1.41
23.3 (32.6) 1.45
21.8 (35.5) 1.41
23.1 (39.2) 1.48
22.9 (40.7) 1.56
26.2 (46.4) 1.22
6.39 (8.2) 16018
7.22 (10.1) 11602
6.80 (10.1) 13577
6.33 (10.1) 13641
6.42 (10.1) 16894
6.53 (10.2) 15062
6.86 (10.2) 18819
6.71 (10.2) 14607
377, 397 (77 K) ꢀ0.01
359, 379 (77 K)
361, 377 (Fꢀ)
362, 378 (Fꢀ)
382 (Fꢀ)
0.31
0.15
ꢀ0.02
ꢀ0.13
ꢀ0.20
ꢀ0.20
ꢀ0.12
22.3
(41.7) 1.45
352, 365 (Fꢀ)
362, 376 (Fꢀ)
392 (Fꢀ)
26.9 (46.8) 1.34
23.5 (41.7) 1.13
a
weak/bulk donor/acceptor
[a] Calculated by B3LYP/6-31G. [b] Values for a from Equation (3) with d=1.0; r is the center-to-center dis-
tance of the charges, calculated by B3LYP/6-31G, the distance between a boron atom and the center of a
pyrene unit for PB1–3 or/and a nitrogen atom for other tridurylboranes. [c] Slope of the solvatofluorochromic
plots. [d] Calculated by Equation (1), values in parentheses from the calculation with r instead of a. [e] Poten-
tials versus Fc+/Fc (ferrocenium/ferrocene) in acetonitrile. [f] LE-state emission; “77 K” in parentheses stands
for the spectral data measured in a rigid glass at 77 K; “Fꢀ” indicates fluorescence peak(s) from measurements
(PB1–3 and CBC1) the fluores-
cence spectra display strong LE
emissions at 77 K, and CT emis-
sions at room temperature in
the two solvents except PB1. In
after the compound binds fluoride ions (Figure S1, see the Supporting Information). [g] DG0 =EoxꢀEredꢀDE0,0
.
Chem. Eur. J. 2012, 18, 15512 – 15522
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15515

Q.-H. Song et al.
Figure 4. Fluorescence spectra of the tridurylboranes in two rigid glasses
at 77 K: MTHF (open symbols) and EtOH (full symbols).
the systems with strong donor and/or strong acceptor, there
is no the LE state due to a strong coupling interaction be-
tween the donor and the acceptor. However, their fluores-
cence maxima at 77 K are much shorter than those at room
temperature.
The excited-state interconversions under different condi-
tions
In binary solvent mixtures: In our previous communication,
we have clearly observed interconversions between the LE
state, the ICT state, and the TICT state of PB1–3 in the
binary solvent mixtures (ethanol/glycerol (E/G, v/v 100:0 to
6:94).[18] The two solvents, ethanol and glycerol, were select-
ed because the former possesses a low viscosity and a rela-
tively low polarity (e=24.5) and the latter is a high-viscosity
and highly polar solvent (e=42.5). Upon increasing the pro-
portion of glycerol (G%), both the polarity and the viscosity
of the mixture would increase to result in interconversions
between the excited states.
To investigate the conformational transitions of the excit-
ed states, similar observations were performed for CBC1–3
and CBN3a–c. Figure 5 (top) shows fluorescence spectra of
CBC1 in the binary solvent mixtures. As increasing G% for
the CBC1 system, the CT emission increases accompanying
a redshift, and then gradually decreases and undergoes a
blueshift to form the emission spectrum with a peak at l=
397 nm. When the ratio of E/G is 32:68, the fluorescence
spectrum displays a dual fluorescence emission. These pro-
Figure 5. Fluorescence spectra of CBC1–3 and CBN3a–c in various etha-
~
nol/glycerol (E/G) binary mixtures from 100:0 (&) to 6:94( ) at room
temperature: dual- or tri-fluorescence emission (dashed line), fluores-
cence peaks (open stars). Inset: Dependence of the intensity maxima on
the proportion of glycerol (G%).
creased polarity, and then the CT state converts to the LE
state resulting from a large increase of the viscosity. This im-
plies that the formation of the CT state involves a molecular
conformational change, which is suppressed in high-viscosity
solutions.
The fluorescence spectrum of CBC2 in neat ethanol is
characterized by a single long-band emission with a peak at
l=488 nm (Figure 5). With increasing the proportion of
glycerol, the emission intensity decreases and undergoes a
small redshift to l=492 nm, and then a new short-band
peak at l=430 nm appears. As the long-band emission re-
duces and the short-band emission enhances, an isoemissive
point at l=479 nm was observed. The short-band peak
rises, and then decreases. Obviously, there are two types of
CT states of CBC2 in the binary solvents, that is, the emis-
ACHTUNGTRENNUNGcesses derive from the excited-state interconversion and an
equilibrium between the CT state and the LE state. Namely,
the CT state of CBC1 initial redshifts resulting from an in-
15516
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15512 – 15522

Twisted Donor-Substituted Tridurylboranes
FULL PAPER
sion peak from l=488–492 nm should be assigned to the
TICT emission, and the peak at l=430 nm is assigned to an-
other CT emission. The initial increase of G% causes a red-
shift of the TICT-emission peak resulting from an increase
of the polarity of the solvent, and a further increase of G%
leading to a large increase of the viscosity would suppress
the formations of the first TICT state (decreases and then
disappears) and then the CT state (increase followed by de-
crease). Hence, conformational transitions in the formation
of the two CT states (both TICT and the ICT) occur and are
limited in high-viscosity solutions, and the conformational
transition of the TICT state is suppressed more easily than
that of the ICT state.
A similar change was observed from the fluorescence
spectra of CBC3a (Figure 5). As glycerol is added, the long-
band emission (l=525 nm) decreases and redshifts to l=
536 nm, accompanying a weak LE emission appears. Subse-
quently, a new short-band emission (lmax =463 nm) appears
and rises. When the ratio of E/G is 17:83, the spectrum ex-
hibits a triple fluorescence. Finally, the spectrum shows an
enhanced short-band emission with a peak at l=463 nm
and a weak LE emission. The former process should be a
solvatochromic consequence of the TICT process resulting
from the increased polarity and viscosity of the solvent. The
latter process is the conversion from TICT state to the
short-band CT state. Once again this shows that the forma-
tion of the TICT state is suppressed more easily than the
CT state, that is, the formation of TICT state involves a
higher energy barrier.
6:94 v/v), the LE-to-ICT isoemissive point of PB1 was ob-
served at l=421 nm.[18]
In addition, there is a good correlation between the spec-
tral response and the bulk of a tridurylborane. Namely, the
larger the bulk of the molecule or donor, the more remark-
AHCTUNGTREGaNNNU ble spectral response at lower G% was observed (insets of
Figure 5). For example, comparison of the same series
(PB1–3, CBC1–3a, and CBC3a vs. CBN3a, CBC3b vs.
CBN3b and CBN3c) shows that the larger molecular bulk
(PB1 vs. PB2 and PB3, and CBC1 vs. CBC2 and CBC3a),
the larger donor (CBN3b vs. CBC3b), and more difficult
bond rotation (CBN3a vs. CBC3a, and CBN3c vs. CBC3b)
exhibit more earlier spectral response during an increase of
G%.
At various temperature: In a temperature range from 0–
1508C, the fluorescence spectra in the binary solvents (E/G,
v/v 6:94 for CBC1 and CBC3, 38:62 for CBC2) provided a
further support for the state interconversions (Figure S4, see
the Supporting Information). In these high-viscosity sol-
vents, an elevating temperature results in a notable decrease
in the viscosity, facilitating the conformational transition in
the excited state. Similar temperature effects have been ob-
served from PB1–3 in the binary solvent system.[18]
The excited-state interconversions were further observed
from the fluorescence spectra of single-solvent systems in a
low temperature region (293–77 K), as shown in Figure 6.
At room temperature (293 K), the fluorescence spectra of
The fluorescence change of CBC3b is another mode
(Figure 5). CBC3b in ethanol is almost non-fluorescent.
With increasing the proportion of glycerol, the fluorescence
intensity increases and forms a peak at l=483 nm. This
should be the conversion from the non-fluorescent TICT
state to another CT state because the increased viscosity
suppresses the conformational transition for the TICT state.
In the binary solvent mixtures, the fluorescence change for
CBN3a is similar to those of CBC2 and CBC3a. The fluores-
cence changes of CBN3b and CBN3c are the same mode as
CBC3b.
In the above-described processes with an increase of G%,
there are two kinds of effects: 1) solvatochromic effects re-
sulting from an initial increase in polarity; 2) limitation of
conformational transitions resulting from the markedly in-
creased viscosity. The former causes the conversion from the
LE state to a CT state or from a CT state to a TICT state.
The latter results in a contrary conversion, that is, from a
TICT state to a CT state and then to the LE state. In the
binary solvent mixtures, the fluorescence spectra render the
former effect at low G% and the latter effect at high G%.
In the fluorescence spectra of the tridurylboranes in the
binary solvents, there are isoemissive points (l=437 nm for
PB2, l=483 nm for PB3,[18] l=479 nm for CBC2, and l=
482 nm for CBN3a), which can be interpreted in terms of
two emission species with a parent–daughter relationship as
one of the proofs for the conversion from the TICT state to
the ICT state. In methanol/glycerol binary solvents (100:1–
Figure 6. Fluorescence spectra of PB3 and CBN3b in ethanol and CBC2
~
~
in MTHF recorded at the temperature range of 293 ( )—77 K ( ): fluo-
rescence peaks (open stars).
Chem. Eur. J. 2012, 18, 15512 – 15522
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15517

Q.-H. Song et al.
PB3 in ethanol and CBC2 in MTHF display single long-
band emissions with peaks of l=508 and 456 nm, respec-
tively, and no fluorescence emission for CBN3b in ethanol.
As the temperature decreases, the fluorescence peaks under-
go a redshift to l=518 nm for PB3 and to l=522 nm for
CBC2, and then a blueshift to convert to the LE emission
and increase for PB3, and a rapid blueshift to a short-band
emission for CBC2, lastly, the peak at l=398 nm with a
shoulder at l=359 nm at 77 K, which is the LE emission of
CBC2. In the case of CBN3b, fluorescence peaks appear, in-
crease, and undergo a blueshift. Lastly, the spectrum shows
a single-band emission with a peak at l=416 nm. Clearly,
the state conversions occurred, from the TICT state to the
CT state then the LE state for PB3 and CBC2, and from the
TICT state to the CT state for CBN3b, with decreasing tem-
perature from 293–77 K. These temperature effects may
mainly derive from changes in the polarity and the viscosity
during varying temperature.
For the twisted tridurylboranes, the internal relaxation in
the excited states would occur with larger or smaller twisted
angles (q), as compared to the conformation in the ground
state (see below). Thus, there are two effects in their excited
states when the two conformational changes occur, stabiliza-
tion by the coupling of the D and the A with delocalized
mesomeric effects for the more planar conformational
change, and stabilization by solvation of the dipole for more
twisted ICT with a larger charge separation. These effects
may result in two stable conformers, a more planar ICT
state and a more twisted ICT state as compared to that in
the ground state.
Figure 7. CV diagrams of the tridurylboranes in acetonitrile solutions.
This situation was clarified in the case of the biphenyl de-
rivative, a DMABN analogue, in which two types of CT
states can occur simultaneously.[26] However, two CT states
cannot directly be distinguished from two strong overlap-
ping emission bands. In contrast, our systems clearly exhibit
three types of excited states from their fluorescence spectra.
PB3 or from CBC1 to CBC3, whereas an enhanced elec-
tron-donating ability of the donor would lead to a small in-
crease in ꢀEred, for instance, from CBN3a to CBN3c.
Thermodynamic estimate of a possible ICT process: Because
the formation of the TICT state undergoes an approximate-
ly full charge separation and a decoupling conformational
change, its thermodynamics can be roughly estimated in
terms of an intramolecular electron-transfer process. Hence,
the free energy change (DG) of the proposed electron-trans-
fer reaction is estimated according to the Equation (4),[27]in
which DE0,0 is the energy level of the LE state, r the donor–
acceptor distance, and Ecoul is the Coulomb term.
Thermodynamic estimation of the ICT processes
Redox potentials: In order to estimate the thermodynamics
of the ICT processes of the tridurylboranes, their electro-
chemical properties were examined by the cyclic voltammet-
ric method in acetonitrile solutions with tetrabutylammoni-
um hexafluorophosphate (TBAPF6) as the supporting elec-
trolyte. The cyclic voltammograms of these compounds
show reversible redox waves (Figure 7). The reduction po-
tentials (ꢀEred) decrease markedly in the order of tri-donor
(PB1, CBC1), di-donor (PB2, CBC2) and, mono-donor sys-
tems (PB3, CBC3a,b, and CBN3a–c). The oxidation poten-
tials (Eox) gradually increase in the above-described order
for the same series, and systems with a single, stronger elec-
tron donor exhibit a lower oxidation potential such as
CBC3b versus CBC3a, and from CBN3c to CBN3a. Hence,
an increased electron-accepting ability of the acceptor (i.e.,
a decrease in the ꢀEred values) would lead to a slight de-
crease in the electron-donating ability of the same donor
(i.e., an increase in the Eox values), such as from PB1 to
DG ¼ Eoxꢀ EredþEcoulꢀDE0,0
ꢀ
ꢁ
ð4Þ
e
1
2
Ecoul
¼
ꢀ
4pe0r
e
3:75
Values for DE0,0 are obtained from the emission spectral
data, for instance, the LE-state emissions for PB1–3 and
CBC1 at 77 K, and the fluorescence spectra after the accept-
or p(B) binds fluoride ions to block the ICT process for
other compounds (Figure S1, see the Supporting Informa-
tion).
The values of the free energy change DG would reflect
the driving force of an ICT process. If the Coulomb term is
15518
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15512 – 15522

Twisted Donor-Substituted Tridurylboranes
FULL PAPER
neglected, the calculated values (symbol as DG0, listed in
Table 1) show a gradual decrease from PB1 to PB3, from
CBC1 to CBC3a,b ,and from CBN3a to CBN3c, that is, an
increase in the driving force of the ICT processes. The
system with the smallest driving force, that is, PB1, displays
the LE emission, and weak CT emission in two strongly
polar solvents (acetone and acetonitrile). However, the sys-
tems with large driving force, such as CBC3b, give uniform
CT emissions. This can explain the observed solvatochromic
effects, that is, the larger the driving force of the ICT proc-
esses, the larger the fluorescence redshift, thereby, the
higher the dipole moment (or the higher the extent of
charge separation). A negative DG value implies that a full
charge separation can occur to form the TICT state in a tri-
durylboron molecule.
DG0, such as CBC3b and CBN3c, reveal a contrary trend,
that is, continuing decrease of Ff except in n-hexane for
CBC3b. In short, for tridurylboranes with a larger DG0
value, their Ff maxima were obtained in the more strongly
polar solvent.
The TICT state is expected to be weakly fluorescent, and
the LE emission usually displays a narrow band spectrum
with a relatively low intensity. Thus, the ICT states should
give high fluorescence quantum yields. For this reason, the
fluorescence quantum yields in various solvents can reflect
the excited-state conversions of these compounds. Namely,
with an increase of the solvent polarity, the excited-state
conversions can occur from the LE state to the ICT state,
and/or from the ICT state to the TICT state.
Therefore, three types of excited states of the tridurylbor-
anes, the LE state, the ICT state, and the TICT state, are
characterized by the following properties including confor-
mation, thermodynamics, and photophysics (the fluores-
cence maximum lmax, the bandwidth at half-maximum Dv1/2,
and the fluorescence quantum yield Ff):
Characterization of three types of excited states: The fluo-
rescence quantum yields (Ff) of the tridurylboranes were
measured in various solvents. These values of Ff as a func-
tion of the free energy changes (DG) including the Coulomb
term are shown in Figure 8. With enhancing the driving
1) The LE state with a small driving force (DG>0.5 eV),
the shortest lmax, narrow Dv1/2, relatively low Ff, and a
zero dipole moment, which is independence of the polar-
ity of the solvent and the temperature.
2) The ICT state with a more planar conformation, an inter-
mediate driving force (0.1 eV<DG<0.5 eV), shorter
lmax, narrow Dv1/2, the highest Ff, and an intermediate
dipole moment.
3) The TICT state with a more twisted conformation, a
large driving force (DG<0.1 eV), the longest lmax, the
broadest Dv1/2, very low Ff even no fluorescence, and the
largest dipole moment.
A mechanistic illustration is shown based on the potential
energy, the conformational changes with respect to the inter-
annular twist angle q between the D and the A in the
ground state (S0), and the two CT states, the ICT state and
the TICT state in Figure 9. Thermodynamic estimates of the
CT process reveals that the LE state is often formed in the
systems with small driving forces (DG>0.5 eV, i.e., PB1 and
CBC1). In the tridurylboron molecules with intermediate
Figure 8. Fluorescence quantum yields of the tridurylboranes as a func-
tion of the free energy changes (DG) in a broad range of solvents in the
order of decreasing values of DG: n-hexane, toluene, chloroform, THF,
CH2Cl2, DCE, ethanol, and acetonitrile.
force (i.e., decreasing DG values), the quantum yield in-
creases followed by decreases, and there is a maximum of
Ff at DG approximately 0.3 eV. With an increasing solvent
polarity, the tridurylboranes with a large DG0, such as PB1
and CBC1 exhibit a continuous increase of Ff except in two
strongly polar solvents, and the compounds with a small
Figure 9. Potential energy and conformation diagram of the ground states
(S0) and the excited states for the photochemical reaction of the triduryl-
boranes in various solvents for the weak donor (dash line), the intermedi-
ate (gray bold line), and the strong donor and acceptor (plain line).
Chem. Eur. J. 2012, 18, 15512 – 15522
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15519

Q.-H. Song et al.
driving forces (0.1 eV<DG<0.5 eV), a more planar ICT
state would form through the intermediate balance between
two contrary effects: stabilization of the dipole by solvation
and by enhanced coupling between the D and the A, and
destabilization by the steric hindrance. For the tridurylboron
compounds with large driving forces (DG <0.1 eV) the
TICT process can occur. The formation of two CT states in-
volves conformational changes, and the TICT process with a
higher energy barrier is more easily suppressed than the
conversions. Temperature effects derive from the resulting
changes in solvent polarity and viscosity.
Based on photophysical and photochemical properties
and electrochemical properties, three excited states of the
tridurylboranes were characterized by the properties as fol-
lows:
1) Conformational change, increase in the order LE, ICT,
and TICT; the LE state without conformational change (a
Franck–Condon state), the ICT state with a more planar
conformation, and the TICT state with a more twisted con-
formation as compared to that of the ground state.
2) Thermodynamics, that is, driving forces in the forma-
tion of the excited states increase in the order LE, ICT, and
TICT.
3) Fluorescence properties, the LE state: short, narrow
emissive band, and less emission; the ICT state: short,
narrow emissive band, and strong emission, and the TICT
state: the longest and broadest band, and the least emission.
ICT process, EaACHTUNGTRENNUNG(TICT)>EaAHCTUNGTRENN(UGN ICT). When its conformational
change is limited in the binary solvents, the TICT state is
first suppressed to form the ICT state, which is itself sup-
pressed to form the LE state, as the viscosity increases. For
the system with a large driving force (i.e., CBC3b and
CBN3b,c), there is no LE state due to a strong coupling in-
teraction between the A and the D in the ground state.
With an increase of the solvent polarity, the excited state of
the system may convert from the LE state to the ICT state
and then to the TICT state, and their potential energies
reduce and charge-separation extents increase. In rigid
glasses at 77 K, a conformational change is limited, thus the
systems with a small driving force (PB1–3, CBC1) reveal the
LE-state emission (dashed line in Figure 9), and systems
with a larger driving force, such as CBC3b and CBN3b,c,
display a CT-state emission (plain line in Figure 9).
Experimental Section
Materials and general methods: All materials were obtained from com-
mercial suppliers and used without further purification. Solvents of tech-
nical quality were dried and distilled prior to use.
1
Melting points were uncorrected. H and 13C NMR spectra were recorded
on a Bruker AV spectrometer (300 MHz for 1H, 75 MHz for 13C). FTIR
spectra were carried out on a Bruker Vector22 infrared spectrometer.
UV/Vis absorption spectra were recorded at room temperature with a
Shimadzu UV-2401PC UV/Vis absorption spectrometer. Fluorescence
emission spectra in solution were measured on a Shimadzu RF-5301PC
fluorescence spectrometer except the temperature effects were measured
on a Perkin–Elmer LS55 luminescence spectrometer. Mass spectra were
obtained with a Micromass GCT-TOF or Thermo LTQ Orbitrap mass
spectrometers.
Conclusion
In summary, a series of donor-substituted tridurylboranes
have been synthesized as various twisted D–A molecules
with different donating/accepting ability. Among these twist-
ed D–A molecules, solvatochromic effects show that the ex-
cited states of most compounds possess large dipole mo-
ments, which is evidence for the formation of the TICT
state. Furthermore, the photochemical behavior under dif-
ferent conditions (solvent polarity, solvent viscosity, and
temperature) reveals three types of excited states: the LE
state, the more planar ICT state, and the more twisted TICT
stat. The energy barrier of the conformational transition of
the TICT process is higher than that of the ICT process.
Generally, the excited-state distribution is the LE state or
the ICT state in low-polarity solvents and the TICT state in
strongly polar solvents. Under appropriate conditions, dual
or triple fluorescence emission can be clearly observed, cor-
responding to the simultaneous population of two or three
excited states.
The interconversions between three excited states can
occur upon varying the conditions. The charge-separated
extent and the conformational change in the excited state
increase in the order LE, ICT, and TICT. Thus, the large
charge-separated state would be stabilized by polar solvents,
and the large conformational change would be suppressed
by high-viscosity solvents. For example, increased solvent
polarity can result in conversions of excited states from the
LE state to the ICT state and/or from the ICT state to the
TICT state, and increased viscosity would cause contrary
X-ray diffraction data for 5 were collected at 290(2) K on a Gemini S
Ultra CCD diffractometer (Oxford diffraction Ltd.) equipped with MoKa
monochromated radiation (l=0.71073 ꢁ). CCDC-891943 contains the
supplementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Cyclic voltammetry was carried out on a CHI620D multipurpose electro-
chemical analyzer by using a conventional three-electrode system consist-
ing of a glassy carbon working electrode, a Pt wire electrode, and a satu-
rated calomel reference electrode (SCE). Ferrocene/ferrocenium
(+0.64 V vs. normal hydrogen electrode (NHE)) was used as an internal
standard. All measurements were performed at ambient temperature
under a nitrogen atmosphere in 0.1m solutions tetrabutylammonium
fluorophosphate in acetonitrile at a scan rate of 15 mVsꢀ1
hexaACHTGNUTERNUNNG .
Measurements of the fluorescence quantum yields: The fluorescence
quantum yields (Ff) of the tridurylboranes were determined in various
solvents under aerated solutions with solvent refractive index correction.
The optical density of all solutions (the reference and the tridurylbor-
anes) was about 0.05 at the excitation wavelength. A quinine sulfate
(Ff =0.546 in 0.1n H2SO4)[28] was used as a reference standard. An error
of 10% is estimated for the fluorescence quantum yields.
Calculation methods: The ground-state geometry of the molecules were
first optimized at the AM1 level, and then calculated at the B3LYP/6-
31G level of theory on a PC cluster by using the algorithms supplied with
the package of Gaussian 03 revision B.01.[29]
For the synthesis and the characterization of the tridurylboranes see the
Supporting Information.
15520
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15512 – 15522

Twisted Donor-Substituted Tridurylboranes
FULL PAPER
[9] a) K. A. Zachariasse, T. von der Haar, A. Hebecker, U. Leinhos, W.
Kꢄhnle, Pure Appl. Chem. 1993, 65, 1745–1751; b) T. von der Haar,
A. Hebecker, Y. V. Ilꢈichev, Y.-B. Jiang, W. Kꢄhnle, K. A. Zachar-
iasse, Recl. Trav. Chim. Pays-Bas 1995, 114, 430–442; c) K. A. Za-
chariasse, M. Grobys, T. von der Haar, A. Hebecker, Y. V. Ilꢈichev,
Y.-B. Jiang, O. Morawski, W. Kꢄhnle, J. Photochem. Photobiol. A
1996, 102, 59–70; d) K. A. Zachariasse, M. Grobys, T. von der Haar,
A. Hebecker, Y. V. Ilꢈichev, O. Morawski, I. Rꢄckert, W. Kꢄhnle, J.
Photochem. Photobiol. A 1997, 105, 373–383; e) Y. V. Ilꢈichev, W.
Kꢄhnle, K. A. Zachariasse, J. Phys. Chem. A 1998, 102, 5670–5680;
f) K. A. Zachariasse, Chem. Phys. Lett. 2000, 320, 8–13; g) A. De-
meter, K. A. Zachariasse, Chem. Phys. Lett. 2003, 380, 699–703;
h) K. A. Zachariasse, S. I. Druzhinin, W. Bosch, R. Machinek, J.
Am. Chem. Soc. 2004, 126, 1705–1715; i) S. Techert, K. A. Zachar-
iasse, J. Am. Chem. Soc. 2004, 126, 5593–5600.
[10] a) A. Safarzadeh-Amiri, Chem. Phys. Lett. 1986, 125, 272–278; b) K.
Rechthaler, G. Kçhler, Chem. Phys. Lett. 1996, 250, 152–158; c) N.
Eilers-Kçnig, T. Kꢄhne, D. Schwarzer, P. Vçhringer, J. Schroeder,
Chem. Phys. Lett. 1996, 253, 69–76; d) S. A. Kovalenko, R. Schanz,
T. A. Senyushkina, N. P. Ernsting, Phys. Chem. Chem. Phys. 2002, 4,
703–707.
[11] H. Gruen, H. Gçrner, Z. Naturforsch. A 1983, 38, 928–936.
[12] a) Y. V. Ilꢈichev, W. Kꢄhnle, K. A. Zachariasse, Chem. Phys. 1996,
211, 441–453; b) Y. V. Ilꢈichev, K. A. Zachariasse, Ber. Bunsen-Ges.
Phys. Chem. 1997, 101, 625–635; c) F. D. Lewis, W. Weigel, J. Phys.
Chem. A 2000, 104, 8146–8153.
[13] S. I. Druzhinin, N. P. Ernsting, S. A. Kovalenko, L. P. Lustres, T. A.
Senyushkina, K. A. Zachariasse, J. Phys. Chem. A 2006, 110, 2955–
2969.
[14] a) W. Fuss, K. K. Pushpa, W. Rettig, W. E. Schmid, S. A. Trushin,
Photochem. Photobiol. Sci. 2002, 1, 255–262; b) I. Gꢅmez, M. Reg-
uero, M. Boggio-Pasqua, M. A. Robb, J. Am. Chem. Soc. 2005, 127,
7119–7129.
[15] S. Cogan, S. Zilberg, Y. Haas, J. Am. Chem. Soc. 2006, 128, 3335–
3345.
[16] K. A. Zachariasse, Trends Photochem. Photobiol. 1994, 3, 211–227.
[17] M. Zander, W. Rettig, Chem. Phys. Lett. 1984, 110, 602–610.
[18] M. G. Ren, M. Mao, Q. H. Song, Chem. Commun. 2012, 48, 2970–
2972.
Acknowledgements
We thank the National Natural Science Foundation of China (Grant No.
20972149), the Science Foundation of Anhui Province (1208085M19),
and the Graduate Innovation Fund of the University of Science and
Technology of China (USTC) for financial support, the supercomputing
system in the Supercomputing Center of USTC for facility.
[1] For reviews, see: a) T. W. Hudnall, C. W. Chiu, F. P. Gabbaꢃ, Acc.
Chem. Res. 2009, 42, 388–397; b) Z. M. Hudson, S. Wang, Acc.
Chem. Res. 2009, 42, 1584–1596; c) N. Kitamura, E. Sakuda, Y.
Ando, Chem. Lett. 2009, 38, 938–943; d) C. R. Wade, A. E. J.
Broomsgrove, S. Aldridge, F. P. Gabbaꢃ, Chem. Rev. 2010, 110,
3958–3984.
[2] Samples for recent papers: a) T. Matsumoto, C. R. Wade, F. P.
Gabbaꢃ, Organometallics 2010, 29, 5490–5495; b) C. Baik, S. K.
Murphy, S. Wang, Angew. Chem. 2010, 122, 8400; Angew. Chem. Int.
Ed. 2010, 49, 8224; c) E. Sakuda, Y. Ando, A. Ito, N. Kitamura, J.
Phys. Chem. A 2010, 114, 9144–9150; d) W.-J. Xu, S.-J. Liu, X.-Y.
Zhao, S. Sun, S. Cheng, T.-C. Ma, H.-B. Sun, Q. Zhao, W. Huang,
Chem. Eur. J. 2010, 16, 7125–7133; e) A. Caruso, Jr., J. D. Tovar,
Org. Lett. 2011, 13, 3106–3109; f) C. Sun, J. S. Lu, S. Wang, Org.
Lett. 2011, 13, 1226–1229; g) Z. M. Hudson, S. Wang, Dalton Trans.
2011, 40, 7805–7816; h) C. R. Wade, F. P. Gabbaꢃ, Organometallics
2011, 30, 4479–4481; i) D. Cao, H. Y. Zhao, F. P. Gabbaꢃ, New J.
Chem. 2011, 35, 2299–2305; j) B. A. Blight, A. F. Stewart, N. Wang,
S. Wang, Inorg. Chem. 2012, 51, 778–780; k) Z. G. Zhou, A. Waka-
miya, T. Kushida, S. Yamaguchi, J. Am. Chem. Soc. 2012, 134, 4529–
4532.
[3] J. Feng, K. J. Tian, D. H. Hu, S. Q. Wang, S. Y. Li, Y. Zeng, Y. Li,
G. Q. Yang, Angew. Chem. 2011, 123, 8222–8226; Angew. Chem. Int.
Ed. 2011, 50, 8072–8076.
[4] a) C. Cornelissen-Gude, W. Rettig, J. Phys. Chem. A 1999, 103,
4371–4377; b) R. Stahl, C. Lambert, C. Kaiser, R. Wortmann, R. Ja-
kober, Chem. Eur. J. 2006, 12, 2358–2370; c) U. Megerle, F. Selmai-
er, C. Lambert, E. Riedle, S. Lochbrunner, Phys. Chem. Chem.
Phys. 2008, 10, 6245–6251; d) G. Zhou, M. Baumgarten, K. Mꢄllen,
J. Am. Chem. Soc. 2008, 130, 12477–12484.
[19] S. Yamaguchi, T. Shirasaka, K. Tamao, Org. Lett. 2000, 2, 4129–
4132.
[5] a) K. Rotkiewicz, K. H. Grellmann, Z. R. Grabowski, Chem. Phys.
Lett. 1973, 19, 315–318; b) A. Siemiarczuk, Z. R. Grabowski, A.
[20] H. Suenaga, K. Nakashima, T. Mizuno, M. Takeuchi, I. Hamachi, S.
Shinkai, J. Chem. Soc. Perkin Trans. 1 1998, 1263–1267.
[21] R. Grisorio, G. Melcarne, G. P. Suranna, P. Mastrorilli, C. F. Nobile,
P. Fini, S. Colella, E. Fabiano, M. Piacenza, F. D. Sala, G. Ciccarella,
M. Mazzeo, G. Gigli, J. Mater. Chem. 2010, 20, 1012–1018.
[22] Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T.
Kajita, M. Kakimoto, Chem. Lett. 2008, 37, 262–263.
[23] A. Hameurlaine, W. Dehaen, Tetrahedron Lett. 2003, 44, 957–959.
[24] a) E. Z. Lippert, Elecktrochem. 1957, 61, 962–975; b) N. Mataga, Y.
Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 1956, 29, 465–470; c) W.
Baumann, H. Bischof, J.-C. Frohling, C. Brittinger, W. Rettig, K.
Rotkiewicz, J. Photochem. Photobiol. A 1992, 64, 49–72; d) J.-F.
Lꢉtard, R. Lapouyade, W. Rettig, J. Am. Chem. Soc. 1993, 115,
2441–2447.
[25] a) J.-S. Yang, K.-L. Liau, C.-M. Wang, C.-Y. Hwang, J. Am. Chem.
Soc. 2004, 126, 12325–12335; b) J.-S. Yang, K.-L. Liau, C.-Y. Li, M.-
Y. Chen, J. Am. Chem. Soc. 2007, 129, 13183–13192.
[26] a) M. Maus, W. Rettig, D. Bonafoux, R. Lapouyade, J. Phys. Chem.
A 1999, 103, 3388–3401; b) M. Maus, W. Rettig, Phys. Chem. Chem.
Phys. 2001, 3, 5430–5437; c) M. Maus, W. Rettig, J. Phys. Chem. A
2002, 106, 2104–2111.
´
Krꢅwczynski, M. Asher, M. Ottolenghi, Chem. Phys. Lett. 1977, 51,
315–320; c) Z. R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D. J.
Cowley, W. Baumann, Nouv. J. Chim. 1979, 3, 443–454; d) W.
Rettig, Angew. Chem. 1986, 98, 969–986; Angew. Chem. Int. Ed.
Engl. 1986, 25, 971–988; e) W. Rettig, M. Maus in Conformational
Analysis of Molecules in Excited States (Ed.: J. Waluk), Wiley-VCH,
Weinheim, 2000, Chapter 1, pp. 1–55; f) Z. R. Grabowski, K. Rot-
kiewicz, Chem. Rev. 2003, 103, 3899–4031, and references therein.
[6] a) E. Lippert, W. Lꢄder, F. Moll, W. Nꢆgle, H. Boos, H. Prigge, I.
Seibold-Blankenstein, Angew. Chem. 1961, 73, 695–706; b) E. Lip-
pert, W. Lꢄder, H. Boos in Advances in Molecular Spectroscopy
(Ed.: A. Mangini), Pergamon Press, Oxford, 1962, pp. 443–457.
[7] a) C. J. Jçdicke, H. P. Lꢄthi, J. Am. Chem. Soc. 2003, 125, 252–264;
b) D. Rappoport, F. Furche, J. Am. Chem. Soc. 2004, 126, 1277–
1284; c) I. Gꢅmez, M. Reguero, M. Boggio-Pasqua, M. A. Robb, J.
Am. Chem. Soc. 2005, 127, 7119–7129; d) N. Minezawa, S. Kato, J.
Phys. Chem. A 2005, 109, 5445–5453; e) Y. Amatatsu, J. Phys.
Chem. A 2005, 109, 7225–7235.
[8] a) E. Gilabert, R. Lapouyade, C. Rulliꢇre, Chem. Phys. Lett. 1988,
145, 262–268; b) W. Rettig, W. Majenz, Chem. Phys. Lett. 1989, 154,
335–341; c) R. Lapouyade, K. Czeschka, W. Majenz, W. Rettig, E.
Gilabert, C. Rulliꢇre, J. Phys. Chem. 1992, 96, 9643–9650; d) E.
Abraham, J. Oberlꢇ, G. Jonusauskas, R. Lapouyade, C. Rulliꢇre, J.
Photochem. Photobiol. A 1997, 105, 101–107; e) E. Abraham, J.
Oberlꢇ, G. Jonusauskas, R. Lapouyade, C. Rulliꢇre, Chem. Phys.
1997, 214, 409–423; f) Y. Amatatsu, Theor. Chem. Acc. 2000, 103,
445–450; g) Y. Amatatsu, Chem. Phys. 2001, 274, 87–98; h) D.
Pines, E. Pines, W. Rettig, J. Phys. Chem. A 2003, 107, 236–242.
[27] a) D. Rehm, A. Weller, Isr. J. Chem. 1970, 10, 259–271; b) A.
Weller, Z. Phys. Chem. (Muenchen Ger.) 1982, 133, 93–98.
[28] J. N. Demas, J. A. Crosby, J. Phys. Chem. 1971, 75, 991–1024.
[29] Gaussian 03, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomer-
y, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyen-
gar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Chem. Eur. J. 2012, 18, 15512 – 15522
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15521

Q.-H. Song et al.
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, 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. Fores-
man, 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. Na-
nayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Pittsburgh
PA, 2003.
Received: May 16, 2012
Published online: October 2, 2012
15522
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15512 – 15522