The equilibria and conversions between three excited states: The le state and two charge transfer states, in twisted pyrene-substituted tridurylboranes

Article abstract of DOI:10.1039/c2cc17663g

Excited-state conversions were observed from a series of twisted pyrene-substituted tridurylboranes, corresponding to a locally excited (LE) state, a more planar charge transfer (CT) state with a higher fluorescence quantum efficiency, and a more twisted

Full text of DOI:10.1039/c2cc17663g

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The equilibria and conversions between three excited states: the LE state
and two charge transfer states, in twisted pyrene-substituted
tridurylboranesw
Ming-Guang Ren, Mao Mao and Qin-Hua Song*
Received 7th December 2011, Accepted 13th January 2012
DOI: 10.1039/c2cc17663g
Excited-state conversions were observed from a series of twisted
pyrene-substituted tridurylboranes, corresponding to a locally excited
(LE) state, a more planar charge transfer (CT) state with a higher
fluorescence quantum efficiency, and a more twisted CT state.
Triarylboron p-conjugated systems have received wide interest
in the past decades due to many applications in materials
chemistry as nonlinear optical and charge-transport materials,
emitters in organic light-emitting devices (OLEDs), as well as
selective and sensitive chemosensors for the detection of
Fig. 1 Left: Structures of PB1–3. Right: UV absorption (solid line)
À5
and fluorescence (dashed line) spectra of 10 M triarylboranes PB1–3
1
,2
fluoride and cyanide ions. These intriguing electronic and
photophysical properties of triarylboranes derive from the
presence of the vacant p-orbital on the boron (p(B)) acting
as an excellent electron acceptor in intramolecular charge trans-
fer (ICT). Photoinduced ICT in a strong electron donor (D)–
acceptor (A) substituted conjugated system normally displays a
and pyrene as a comparison, in THF solutions.
pyrene, and their absorption edges increase from PB1 to PB3
(
Fig. 1). This is indicative of the conjugation becoming
stronger from PB1 to PB3. DFT calculation results are similar
5
to the crystal structure of a tridurylborane derivatives, that is,
3
the central boron is completely trigonal planar, and three
duryl groups are arranged in a propeller-like fashion. In
addition, our calculations show the dihedral angles between
the boron plane and the duryl planes decrease gradually from
PB1 to PB3 from 55.9 to 50.11, which is in agreement with
their absorption spectra, while the dihedral angles between the
boron and pyrene planes increase from 31.2 to 45.31 (Table S2w).
Hence, these pyrene-substituted tridurylboranes are pretwisted
D–A molecules.
prominent solvatochromic effect. The representative example is
N,N-dimethylaminobenzonitrile with a dual fluorescence emission,
in which the long-band wavelengths strongly depend on solvent
polarity. Since the first observation by Lippert et al., a large variety
of mechanisms have been presented in the literature to explain
3
this phenomenon. Among these, the TICT model has gained
widespread acceptance and appears to be conclusion of the
numerous debates over the past decades. However, there are
still some points that arise leading to doubt or contrary
interpretation. For example, both the twisted angle necessary
The fluorescence spectra of PB1–3 in THF reveal a difference
in the excited-state properties of these molecules, shown in
Fig. 1. Similar to free pyrene, the structured spectrum with
two peaks at 378 and 397 nm from PB1 should be assigned to
the locally excited (LE) state, which is independent of solvent
polarity (vide infra). Both PB2 and PB3 reveal a single-band
emission, which gradually red-shift relative to PB1. This shows
that excited-state energy levels of these compounds from PB1 to
PB3 are reduced, in other words, there is a gradual increase in
charge-separation degree or dipole moment.
3
and excited-state CT equilibria are not quite clear. In this
work, we have observed three excited states from a series of
synthesized pyrene-substituted tridurylboranes PB1–3 (Fig. 1,
for synthesis details see ESIw), and gained some new insights
3
a,4
into the ICT in a pretwisted D–A system.
By comparing with free pyrene, the absorption spectra of
PB1–3 show that the electron coupling between the pyrene
moiety (D) and p(B) (A) unit is very weak. The absorption
maxima of PB1–3 show only small red-shifts relative to free
To obtain dipole moments of the excited states, fluorescence
spectra of these compounds in various polarity solvents were
recorded and are shown in Fig. 2. In most solvents, fluorescence
spectra of PB1 exhibit the LE-state emission, and a new single
long-band emission appears only in two strongly polar solvents,
acetone and acetonitrile. This shows clearly the excited-state
conversion of PB1 from the LE state to an ICT state. However,
the fluorescence spectra of PB2 and PB3 exhibit single band
Department of Chemistry, Joint Laboratory of Green Synthetic
Chemistry, University of Science and Technology of China,
Hefei, 230026, P. R. China. E-mail: qhsong@ustc.edu.cn;
Tel: 86 551 3607992
w Electronic supplementary information (ESI) available: Experimental
details, characterization data and NMR spectra of new compounds,
calculation details of dipole moments, DFT calculated structures and
photographs of solutions of PB1–3. See DOI: 10.1039/c2cc17663g
2
970 Chem. Commun., 2012, 48, 2970–2972
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Fig. 2 Normalized fluorescence spectra of PB1–3 in solvents with
various polarity: 1, n-hexane, 2, toluene, 3, dioxane, 4, chloroform,
Fig. 3 Fluorescence spectra of PB1–3 in various ethanol–glycerol
binary mixtures (100 : 0 to 6 : 94, v/v) at room temperature.
2 2
5, THF, 6, CH Cl , 7, DCE, 8, ethanol, 9, acetone, 10, acetonitrile.
Inset: Fluorescence quantum yields of PB1–3 in the ten solvents above.
of PB1–3 were measured in ethanol–glycerol (E–G) solvent
mixtures, the ratio of E : G ranging from 100 : 0 to 6 : 94 (Fig. 3).
For the PB1 system, with increasing proportion of glycerol
emission except for PB2 in nonpolar hexane, and large solvato-
chromic shifts with increasing solvent polarity. Obviously, a big
difference in the dipole moment between the excited state and the
ground state, and the CT state with a larger polarity is stabilized
(
Fig. 3 top), the LE-state emission increases followed by a
decrease, and a new long-band emission with a peak at 430 nm
appears and rises and then decreases. These processes derive
from the excited-state equilibrium between the LE state and
the ICT state. Namely, the LE state of PB1 converts to the
ICT state with increasing the polarity of solvent mixtures.
When the ratio of E : G is lower than 28 : 72, the ICT-state
emission starts to decrease. This may be because the formation of
the ICT state involves a molecular conformational change, and the
bond rotation would be suppressed in a high-viscosity solution.
The fluorescence spectrum of PB2 in neat ethanol is
characterized by a single long-band emission with a peak at
e
by polar solvents. The dipole moment (m ) of the fluorescent states
can be estimated from the energies of the fluorescence maxima (v
6
f
)
against the solvent parameter Df (for details see ESIw), and are
listed in Table 1.
The dipole moments of the excited-states PB2 and PB3 are
1
8.3 and 21.3 D, respectively. The large dipole moments of
PB2 and PB3 are one of evidences for the formation of the
1
c,3a,b,d,7
TICT state.
The formation of a TICT state would
generate a significant charge separation between the D–A pair,
which is a nearly electronically decoupled D–A pair with
forbidden radiative transition to the ground state, thus a small
TICT fluorescence quantum yield is expected. Hence, change
4
57 nm (middle of Fig. 3). With increasing the proportion of
glycerol, a LE emission appears and rises. The long-band peak
shifts to 470 nm, and then decreases accompanied by a new
peak at 423 nm. The new peak rises followed by a decrease and
eventually disappears. When E : G is 6 : 94, there are only the
LE-emission peaks. Besides the LE state, there are two
CT states of PB2 in the binary solvents. The emission peak
from 457–470 nm may be assigned to the TICT-state emission.
The peak at 423 nm may be assigned to the ICT-state emission.
With increasing the proportion of glycerol, the increase in solvent
polarity results in initial red-shifts of the TICT-emission peak, and
the increase in solution viscosity would suppress the formations of
the TICT state and then the ICT state, because conformational
transitions in the formation of the excited states (TICT and ICT)
are limited in high-viscosity solutions. Obviously, the formation of
TICT state is suppressed more easily than the ICT state.
f
trends of fluorescence quantum yields (F ) of PB2 and PB3
should be another evidence (Fig. 2 insets).
In order to estimate thermodynamics of the ICT processes,
redox potentials of these compounds were measured by cyclic
voltammetry, and data are listed in Table 1. The driving forces of
the processes, free energy change (DG), were estimated (Table 1).
Obviously, the driving force increases gradually from PB1 to PB3.
To investigate the conformational change of the CT states,
we selected two solvents, glycerol with high viscosity and high
polarity (e, 42.5) and ethanol with low viscosity and relatively
low polarity (e, 24.5) for binary solvents. The fluorescence spectra
Table 1 Ground- and excited-states of dipole moments, redox potentials
and free energy changes of the ICT process, of tridurylbonanes PB1–3
The fluorescence spectrum of PB3 in 100% ethanol reveals a
single long-band emission with a peak at 496 nm (Fig. 3 bottom).
When glycerol is added, weak LE-state emission appears, and the
long-band emission rises and shifts to 507 nm (E : G = 12 : 88), and
then decreases and a new short-band emission (lmax = 440 nm)
appears. When the ratio of E : G is 10 : 90, the spectrum
exhibits a triple fluorescence emission. Eventually, there is
a
b
b
c
Compd
m
g
/D
m
e
/D
E
ox /V
ÀEred /V
2.26
2.04
1.81
DG /eV
PB1
PB2
PB3
0.125
5.37
5.37
/
1.37
1.43
1.47
0.35
0.19
0.00
18.3
21.3
a
b +
Calculated by B3LYP/6-31G. vs. Fc /Fc in CH
c
2
Cl
2
.
DG = Eox
À
E
red À DE0,0, DE0,0 from the LE-emission peak, 378 nm.
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Chem. Commun., 2012, 48, 2970–2972 2971

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Fig. 5 Potential energy diagram of ground and excited states for the
photoreaction of tridurylboranes in various polarity solvents.
ICT state to TICT state for PB3. Obviously, with increasing
temperature, decreased viscosity allows the conformational
transition to both CT states.
In summary, three synthesized pyrene-substituted tridurylboranes
are twisted D–A structures. These molecules reveal three excited
states: the LE state, the ICT state and TICT state, which possess
different molecular structures, the LE state with a zero dipole
moment, more planar ICT state and more twisted TICT state,
respectively. The formation of the TICT state with a higher energy
barrier is suppressed more easily than the ICT state. A mechanistic
scheme is shown in Fig. 5, and more support is being collected.
The equilibria and conversions between these states be altered
through varying the polarity and viscosity of the media
and temperature. These distinct properties would have new
potential applications in materials such as fluorescent thermo-
Fig. 4 Fluorescence spectra of PB1–3 in ethanol–glycerol solvent
mixtures (v/v 32 : 68 for PB1, 6 : 94 for PB2 and PB3) at 0–150 1C.
enhanced short-band emission, with a peak at 440 nm, and weak
LE emission. The former process should be a solvatochromic
consequence of a TICT process resulting from increased solvent
polarity. The latter process (blue-shift) is a transformation from the
TICT state to the ICT state. This shows again that the formation of
the TICT state is suppressed more easily than the ICT state, and
this implies that the former has a higher energy barrier.
8
meters, and fluid viscosity indicators.
In the above processes, there are two kinds of effects, one is
from polarity when increasing initially the proportion of
glycerol, and another is from viscosity at high levels of
glycerol. In fluorescence spectra of tridurylboranes in binary
solvents, there are isoemissive points, 437 nm for PB2 and 483 nm
for PB3, which can be interpreted in terms of two emission species
with a parent–daughter relationship as a proof for conversion
from the TICT state to the ICT state. In methanol–glycerol
binary solvents 100 : 0 to 6 : 94 v/v), the LE-to-ICT iso-emissive
point of PB1 was observed at 421 nm (Fig. S3, ESIw).
This work was supported by the National Natural Science
Foundation of China (Grant No. 30870581, 20972149).
Notes and references
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1
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In the temperature range 0 to 150 1C, fluorescence spectra in
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from the LE state to the ICT state for PB1, from the LE state
to the ICT state and then TICT state for PB2, and from the
1
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972 Chem. Commun., 2012, 48, 2970–2972
This journal is c The Royal Society of Chemistry 2012