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
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
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15512 – 15522