Excited-state dynamics of bis(9-fluorenyl)methane and its derivative 9-(9-ethylfluorenyl)-9′-fluorenylmethane: Steric effect on energetics and dynamics of ground- and excited-state conformations

Article abstract of DOI:10.1021/jp411878r

Intramolecular excimer formation of bis(9-fluorenyl)methane (BFM) and 9-(9′-ethylfluorenyl)-9-fluorenylmethane (EFFM), in which an ethyl group is substituted to a 9-H atom in BFM, was studied by means of steady-state and time-resolved fluorescence. Ab initio and DFT calculations enabled the prediction of three conformers as stable species of orthogonal, trans-gauche, and gauche-gauche. The theoretical and experimental results reveal that the substitution effect is also found to appreciably influence the energies, spectroscopy, and kinetics associated with the interconversion of various conformers of the diaryl compounds. We have not observed the rising components in the excimer fluorescence decay of BFM and EFFM in PMMA as observed in the liquid solutions probably because of the existence of the sandwich conformer responsible for the excimer fluorescence prior to the laser irradiation.

Full text of DOI:10.1021/jp411878r

Article
pubs.acs.org/JPCA
Excited-State Dynamics of Bis(9-fluorenyl)methane and its Derivative
9
‑(9-Ethylfluorenyl)-9′-fluorenylmethane: Steric Effect on Energetics
and Dynamics of Ground- and Excited-State Conformations
,
†
‡
†
§
Bong Hyun Boo,* Minyung Lee, Ki-Seok Jeon, and Seung-Joon Kim
†
Department of Chemistry, Chungnam National University, Daejeon 305-764, Republic of Korea
‡
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea
§Department of Chemistry, HanNam University, Daejeon 300-791, Republic of Korea
*
S Supporting Information
ABSTRACT: Intramolecular excimer formation of bis(9-fluorenyl)methane (BFM) and
-(9′-ethylfluorenyl)-9-fluorenylmethane (EFFM), in which an ethyl group is substituted
9
to a 9-H atom in BFM, was studied by means of steady-state and time-resolved
fluorescence. Ab initio and DFT calculations enabled the prediction of three conformers as
stable species of orthogonal, trans−gauche, and gauche−gauche. The theoretical and
experimental results reveal that the substitution effect is also found to appreciably
influence the energies, spectroscopy, and kinetics associated with the interconversion of
various conformers of the diaryl compounds. We have not observed the rising components
in the excimer fluorescence decay of BFM and EFFM in PMMA as observed in the liquid
solutions probably because of the existence of the sandwich conformer responsible for the
excimer fluorescence prior to the laser irradiation.
INTRODUCTION
fluorescence, the dual rise times of the excimer fluorescence, and
■
DFT and MP2 calculations led to a conclusion that three
conformers of BFM are involved in the two independent excimer
Interchromophore separation and dihedral angle which two
aromatic planes constitute in a given molecule are considered to
be prominent in the feasibility of the intramolecular excimer
formation and the enhancement of excimer fluorescence
4
formation processes. The shorter decaying (rising) component
of the monomeric (excimer) fluorescence was attributed to the
excimer formation from the near face-to-face conformer, whereas
the slower decaying (rising) component of the monomeric
1
−13
intensity.
An interesting example of diarylalkanes capable
of forming an intramolecular excimer is bis(9-fluorenyl)methane
BFM), the molecular model of which is shown in Figure 1a.
Boo and his co-workers have shown that BFM exhibits weak
excimer fluorescence in solution at ambient temperature.
observation of triple exponential decay times of the monomeric
(
excimer) fluorescence was attributed to the excimer formation
(
from the orthogonal (og) and trans−gauche (tg′) conformers.
The activation energy for the transformation of the og or tg′
conformer to the sandwich conformer was reported to be 15 kJ/
4
,6,7
The
4
mol by members of our laboratory.
By combining the theoretical, spectroscopic, and kinetic
methods, we wish to elucidate the ground-state thermodynamic
stability and the excited-state dynamics corresponding to the
interconversion of the og, tg′, and gg′ conformers. The major
goals of the present study are (1) to theoretically predict the
ground-state geometries and energies of the various conformers
of BFM and EFFM with the aid of the DFT and MP2 methods,
and then to clarify how much dispersion can affect the geometries
and energies of the diaryl compounds; (2) to clarify the MOs and
symmetries associated with the normal and excimer fluores-
cence; (3) to elucidate the steric effect on the geometries and
energies of the diaryl compounds in the electronically ground
state by substituting a 9-H atom in BFM with an ethyl group (the
Received: December 4, 2013
Revised: February 20, 2014
Published: February 21, 2014
Figure 1. Molecular models of (a) bis(9-fluorenyl)methane (BFM) and
(
b) 9-(9-ethylfluorenyl)-9′-fluorenylmethane (EFFM).
©
2014 American Chemical Society
2269
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The Journal of Physical Chemistry A
Article
Table 1. Electronic, Zero-Point, and the Relative Energies of the Near-Orthogonal, Trans−Gauche, and Near-Sandwich
a
Conformations of Bis(9-fluorenyl)methane Calculated at the Various Levels and by Using the Various Basis Sets
therm. corr. to energy at
therm. corr. to Gibbs free energy at
b
c
d
conformer
orthogonal
sym.
state
electronic energy (au)
B3LYP/cc-pVTZ
zero-point energy (au)
298.15 K (kJ/mol)
298.15 K (kJ/mol)
B3LYP/cc-pVTZ
//B3LYP/cc-pVTZ
B3LYP/cc-pVTZ
//B3LYP/cc-pVTZ
B3LYP/cc-pVTZ
//B3LYP/cc-pVTZ
/
/B3LYP/cc-pVTZ
C2
global
minimum
−1041.297155
0.370673
0.370656
0.370275
51.3
48.7
48.6
−127.9
−121.9
−118.8
e
(
0)
0)
f
(
trans−
Cs
transition
−1041.293378
gauche
(
(
7.3)
15.9)
gauche−
C2v transition
−1041.286867
gauche
(
(
23.3)
35.1)
MP2/6-311++G(d,p)
B3LYP/cc-pVTZ
//B3LYP/cc-pVTZ
B3LYP/cc-pVTZ
//B3LYP/cc-pVTZ
B3LYP/cc-pVTZ
//B3LYP/cc-pVTZ
/
/B3LYP/cc-pVTZ
orthogonal
C2
Cs
global
minimum
−1038.073902
0.370673
0.370656
0.370275
51.3
48.7
48.6
−127.9
−121.9
−118.8
(
(
9.7)
0)
trans−
transition
−1038.074208
gauche
(
(
6.3)
5.2)
gauche−
C2v transition
−1038.076161
gauche
(
(
0)
2.1)
MP2/6-311++G(d,p)
/MP2/6-31G(d)
MP2/6-31G(d)
//MP2/6-31G(d)
MP2/6-31G(d)
//MP2/6-31G(d)
MP2/6-31G(d)//MP2/6-31G(d)
/
orthogonal
C2
Cs
C2
local
minimum
−1038.078223
0.384989
0.384052
0.383210
0.382776
54.1
54.6
54.3
52.9
−127.8
(
(
60.3)
54.9)
trans−
local
minimum
−1038.079900
−131.6
−122.2
−124.9
gauche
(
(
54.0)
44.2)
gauche−
global
minimum
−1038.099493
gauche
(
(
0)
0)
gauche−
C2v transition
−1038.089452
gauche
(
(
23.8)
22.5)
a
Zero-point energies and thermal energies (excluding ZPE) were corrected to the electronic energies as Etotal = E + ZPE + E and to the Gibbs free
el
th
b
c
energies as Gtotal = E + ZPE + E + pV − TS ≡ E + ZPE + G . The ZPE for the DFT calculations was scaled by 0.963. The thermal energy is the
sum of translational, rotational, and vibrational energies at 298.15 K excluding the ZPE. Contribution from translational, rotational, and vibrational
energies at 298.15 K (except the ZPE), Δ(pV)(= RT), and −TS where T = 298.15 K and S = S
in the first parentheses refer to the relative total energies in kilojoules per mole. The values denoted in the second parentheses refer to the relative
el
th
el
th
d
e
, i.e., G = E + pV − TS. The values denoted
298.15 K
th
th
f
Gibbs free energies in kilojoules per mole.
molecular model for the BFM analog is shown in Figure 1b); (4)
to understand the transformation dynamics by measuring steady-
state and time-resolved fluorescence of the diaryl compounds;
and finally (5) to understand the excited-state dynamic motion of
the various conformers of the diaryl compounds in various
solvents with different viscosity. In this paper, we report quantum
chemical, spectroscopic, and kinetic studies concerning the
interconversion of various conformers of BFM and EFFM, which
presumably yield information concerning the nature of the
excited-state conformational changes involved in the formation
of an intramolecular excimer.
EXPERIMENTAL AND THEORETICAL
METHODOLOGIES
■
Synthesis of 9-Chloromethylfluorene. The compound
was synthesized by using a modified Wawzonek and Dufek
14
method. A mixture of 15 g (76 mmol) of 9-fluorenylmethanol
2
and 45 mL (6.2 × 10 mmol) of SOCl was refluxed for 4 h.
2
2
270
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The Journal of Physical Chemistry A
Article
Removal of excess SOCl via a simple distillation followed by a
with the cc-pVTZ19−22(for BFM) or 6-311+G(d) (for EFFM)
basis sets. We calculated the single-point energies of the various
conformers with the MP2/6-311++G(d,p)//B3LYP/cc-pVTZ
(for BFM) and MP2/6-311+G(d)//B3LYP/6-311+G(d) (for
EFFM) methods. The zero-point corrections were made to the
single-point electronic energies by using a transparent scaling
factor of 0.963; the uniform scaling factor in the frequency
2
vacuum distillation yielded 14.5 g (68 mmol) of the crude
product of 9-chloromethylfluorene. Recrystallization over
ethanol afforded 2.8 g (yield =17%) of white crystalline solid
of 9-chloromethylfluorene (measured mp, 67−68 °C; reported
14
value, 66.5−67.5 °C). The purity was checked by means of
thin-layer chromatography (TLC) and gas chromatography
(
GC). The product was identified from IR spectra in which
calculation of various organic compounds was evaluated
−1
23,24
various stretching frequencies corresponding to 3000 cm due
previously with the B3LYP/6-31G(d) method.
Thermal
−1
to aromatic C−H stretching and 1440 cm due to aromatic C
corrections were also made to the single-point electronic
energies and Gibbs free energies.
C stretching appear.
Synthesis of 9-(9-Ethylfluorenyl)-9′-fluorenylmethane
Second, for considering the electron correlation effect
involving dispersion on the structures and energies, we also
reoptimized the geometries and then achieved vibrational
frequencies by using the MP2/6-31G(d) (for BFM) and MP2/
6-31G (for EFFM) methods. We also calculated the single-point
energies of the various conformers with the MP2/6-311+
+G(d,p)//MP2/6-31G(d) (for BFM) and MP2/6-311+G(d)//
MP2/6-31G (for EFFM) methods. The zero-point and thermal
corrections were also made to the single-point electronic
energies. The thermal corrections were also made to the
single-point Gibbs free energies.
(
EFFM). EFFM was synthesized by using a modified Wawzonek
14
and Dufek method. In a 250 mL three-necked round-bottomed
flask equipped with a gas inlet tube and a gas outlet, was placed
.5 mL(35 mmol) of 9-ethylfluorene and 30 mL of petroleum
ether. A mixture of 35 mL of 1.6 M BuLi in hexane (56 mmol)
and 20 mL of petroleum ether were added dropwise to the
reaction mixture. After the reaction mixture was stirred for 10 h at
6
room temperature with N bubbling in, the color of the reaction
2
mixture turned from ivory to red. After the reaction mixture was
stirred for an additional 3 h, an appreciable amount of yellow
solid appeared, and then the color of the solution turned orange.
The harmonic vibrational frequencies were also used to
determine whether a given structure is a local minimum on the
potential energy surface or not. Note that inner-shell electrons
were excluded in the electron-correlation calculation to reduce
6
.4 g (30 mmol) of 9-chloromethylfluorene in petroleum ether
was added dropwise to the reaction mixture. The reaction
mixture was stirred for 13 h. Removal of insoluble salt of LiCl (by
filtration) and volatile liquid afforded a semisolid material.
Sublimation and recrystallization of the product mixture over
ethanol yielded 48 mg of EFFM (yield, 0.43%). The purity was
checked by TLC, NMR, and time-of-flight (TOF) mass
spectroscopy. The purity was determined to be 94% from
TOF mass spectroscopy. The impurity was found to be only 9-
pentylfluorene with 6% formed via a coupling reaction between
n-butyllithium and 9-chloromethylfluorene. The diaryl com-
2
5−27
the computational time. The TD DFT method
was also
applied to compute the vertical excitation energies and oscillator
strengths of the various conformers with the B3LYP/cc-pVTZ//
MP2/6-31G(d) (for BFM) and B3LYP/6-311+G(d)//MP2/6-
31G (for EFFM) methods. Previous studies indicate that for
molecules with low-lying excited states, TD DFT constitutes a
considerable improvement over HF-based methods such as the
2
8−31
single-excitation configuration interaction (CIS).
All
1
calculations were carried out with the Gaussian 09 program
pound was identified from H NMR and GC mass spectra. The
32
suite.
additional support for the mass and structure of the diaryl
compound is given by resonant two-photon ionization (R2PI)
and R2PI TOF mass spectra of the thermal decomposition
products of EFFM, 9-methylfluorene, and 9-ethylfluorene
formed at 180 °C in a nozzle utilized in a molecular beam
arrangement. The TOF and R2PI spectra are available in
Supporting Information as indicated below.
Spectroscopic Method. Fluorescence lifetime was meas-
ured by a time-correlated single-photon counting (TCSPC)
system. A Coherent Antares Nd:YAG laser was mode-locked at
RESULTS AND DISCUSSION
Structures and Energetics. As shown in Table 1, the
■
B3LYP/cc-pVTZ//B3LYP/cc-pVTZ calculation of the energies
of the various conformations of BFM molecule predicts that the
orthogonal conformer is the lowest in the total energy, whereas
the sandwich conformation is the highest. Note that the
orthogonal conformation is achieved as the lowest conformer
during the optimization of the trans−trans conformation as the
promising global minimum. Note that a symmetry constraint
7
6 MHz and frequency doubled by a KDP crystal for
synchronously pumping of a dual-jet dye laser. The cavity-
dumped pulse from the dye laser was 1−2 ps, 100−200 mW
average power at 3.8 MHz dumping, and 560−607 nm tunability
when Rh6G for gain dye and DODCI for saturable absorber were
used. For excitation of the BFM and EFFM compounds, the dye
laser pulse was frequency-doubled by using a KDP crystal. The
instrument response function was measured by scattering the
laser from pure n-pentane, and it was found to have a typical full
width at half-maximum (fwhm) of 0.7 ns when a Hamamatsu
R928 PMT was used. All the standard electronics for the TCSPC
were from EG&G Ortec.
using C is imposed in the optimization of the orthogonal
2
conformer. No imaginary frequency is found in the frequency
calculation of the orthogonal conformation. We found another
low-lying conformer, named as a tg′, which was the second
lowest energy listed in Table 1. Note that a symmetry constraint
using C is imposed in the optimization of the tg′ conformer.
s
−1
However, one imaginary frequency of 5 cm corresponding to
torsional vibration of the fluorenyl groups around the C−C
single bond is found in the frequency calculation by using the
B3LYP/cc-pVTZ//B3LYP/cc-pVTZ method. Hence, we pre-
sume the tg′ conformer with a C is a transition state. The
s
Theoretical Methods. The ground-state equilibrium geo-
imaginary frequency is, however, removed when the C symmetry
s
metries and electronic and vibrational energies of the diaryl
constraint is relieved.
15
compounds were probed by using the Kohn−Sham DFT and
MP2 methods. First, for the DFT calculation, Becke’s three-
We also optimized a gg′ conformer by using the B3LYP/cc-
pVTZ method by imposing a C symmetry. It is also found in the
2
v
1
6,17
parameter exchange functional
and the gradient-corrected
calculation that the two fluorenyl moieties in the face-to-face
conformation are more widened, and not parallel, than that
18
Lee−Yang−Parr correlational functional (B3LYP) were used
2
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The Journal of Physical Chemistry A
Article
Table 2. Electronic, Zero-Point, and the Relative Energies of the Near-Orthogonal, Trans−Gauche, and Near-Sandwich
Conformations of 9-(9′-Ethylfluorenyl)-9-fluorenylmethane Calculated at Various Levels and by Using Various Basis Sets
a
therm. corr. to energy at
298.15 K (kJ/mol)
therm. corr. to Gibbs free energy at
298.15 K (kJ/mol)
conformer sym.
state
electronic energy (au)
B3LYP/6-311+G(d)
zero-point energy (au)
B3LYP/6-311+G(d)
//B3LYP/6-311+G(d)
B3LYP/6-311+G(d)
//B3LYP/6-311+G(d)
B3LYP/6-311+G(d)
//B3LYP/6-311+G(d)
/
/B3LYP/6-311+G(d)
orthogonal
C1 local
−1119.799115
0.424132
0.423780
0.424117
59.2
59.3
58.7
−136.6
−137.6
−134.1
minimum
(
(
20.5)
21.5)
trans−
Cs
global
minimum
−1119.806603
gauche
(
(
0)
0)
gauche−
C1 local
minimum
−1119.799195
gauche
(
(
19.7)
23.8)
MP2/6-311+G(d)
/B3LYP/6-311+G(d)
B3LYP/6-311+G(d)
//B3LYP/6-311+G(d)
B3LYP/6-311+G(d)
//B3LYP/6-311+G(d)
B3LYP/6-311+G(d)
//B3LYP/6-311+G(d)
/
orthogonal
C1 local
minimum
−1116.294351
0.424132
0.423780
0.424117
59.2
59.3
58.7
−136.6
−137.6
−134.1
(
(
41.4)
38.3)
trans−
Cs
local
minimum
−1116.306713
gauche
(
(
8.1)
4.0)
gauche−
C1 global
minimum
−1038.309905
gauche
(
(
0)
0)
MP2/6-311+G(d)
/MP2/6-31G
MP2/6-31G//MP2/6-31G
0.436199
MP2/6-31G//MP2/6-31G
63.1
MP2/6-31G//MP2/6-31G
/
orthogonal
C1 local
minimum
−1116.294222
−139.3
(
(
65.6)
60.6)
trans−
Cs
local
minimum
−1116.304671
0.435170
0.435053
63.8
63.1
−140.3
−134.3
gauche
(
(
36.2)
29.4)
gauche−
C1 global
minimum
−1116.318069
gauche
(
(
0)
0)
a
The notations described here are the same as indicated in Table 1.
expected in the normal propanelike structure probably because
the sandwich conformer, whose energetic behavior is expected to
enhance the excimer fluorescence intensity? We elucidated the
steric effect on the orientations and energies of the diary
compounds in the electronically ground state. As listed in Table
2, the B3LYP/6-311+(d)//B3LYP/6-311+(d) calculation on
EFFM shows that the sandwich conformation is still the highest
in total energy and Gibbs free energy but reduces the energy and
Gibbs free energy differences. Note that no symmetry constraint
was imposed in the optimization of the gg′ conformer of EFFM.
The single-point energy calculation on EFFM with the MP2/6-
311+(d)//B3LYP/6-311+(d) method, however, indicates that
the gg′ conformer is the lowest in both energy and Gibbs free
energy.
of the repulsion between the fluorenyl subgroups. One imaginary
−1
frequency of 17 cm corresponding to a twisting motion of the
two fluorenyl moieties is observed in the frequency calculation.
This indicates that the C optimized structure is a transition-
2
v
state. The single-point energy calculation on BFM by using the
MP2/6-311++G(d,p)//B3LYP/cc-pVTZ method, however,
reveals that the three conformations are almost thermoneutral.
The energetics may arise from the inclusion of the London
dispersion force, a kind of noncovalent interaction between the
fluorenyl moieties. The density matrix analysis on the gg′
conformer by using the TD DFT B3LYP/cc-pVTZ//B3LYP/cc-
pVTZ method indicates there is a slight density matrix element
between a C atom in one part and opposite C′ atoms of the other
part, supporting the noncovalent bonding between the fluorenyl
moieties.
In the MP2 optimizations on BFM and EFFM by using the 6-
31G(d) and 6-31G basis sets, respectively, however, we observe
unexpected energetics that the gg′ geometries with C (BFM)
2
Does an ethyl group substitution to a 9-H atom in BFM
significantly reduce relative internal and Gibbs free energies of
and C (EFFM) are achieved as the lowest conformations. No
1
imaginary frequencies are observed in the vibrational calculations
2
272
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The Journal of Physical Chemistry A
Article
Figure 2. Three possible conformers of (a) bis(9-fluorenyl)methane (BFM) and (b) 9-(9-ethylfluorenyl)-9′-fluorenylmethane (EFFM) optimized with
the MP2/6-31G(d) (for BFM) and MP2/6-31G (for EFFM) methods.
of all the geometries except the C2v gg′ conformer of BFM,
indicating that all the conformations as seen in Figure 2 are global
or local minima in the potential energy surfaces of the diary1l
compounds. Note that one imaginary frequency of 34.8 cm−
corresponding to the twisting mode is observed, indicating that
the C conformer of BFM is a transition state.
Here, we note that the gg′ conformations optimized with the
MP2 methods are stable ones, in which the fluorenyl
chromophores in the gg′ geometries are slightly parallel to
each other, twisted, and less widened than those optimized with
the DFT methods. The NMR data of EFFM listed in
Experimental and Theoretical Methodologies also indicates
along the chromophore axis in addition to the twisting motion
around alkane skeletal bonds, the motions are forbidden in BFM
because of the rigid sp framework. Therefore, it is expected that
3
flexibility for excimer formation of BFM is low compared to that
of 1,3-DNP. In the case of 1,3-DNP, three stable conformations
of tt′, tg′, and gg′ exist in the ground state, indicating that the tt
and tg′ conformations are found to have 57 and 40% outstanding
2
v
9
propensity, respectively.
Spectroscopy and Kinetics. Figure 3 presents the
absorption spectrum of a 1.0 × 10− M BFM solution in
cyclohexane and comparison with the simulated absorption
spectrum achieved with the TD DFT B3LYP/cc-pVTZ
calculation in which 20 vertical excitation energies were
calculated.
5
that the central two protons in the CH group are slightly
2
different in the chemical shifts (δ = 2.50 and δ = 2.54) and the
coupling constants (J = 4.74 Hz and J = 4.71 Hz). This can be
Figure 4 also depicts the absorption spectrum of a 1.2 × 10−
M EFFM solution in cyclohexane and comparison with the
simulated absorption spectrum evaluated with the TD DFT
B3LYP/6-311+G(d)//MP2/6-31G calculation in which 20
vertical excitation energies were calculated.
4
taken as an indication that the C tg′ conformer is not abundant.
s
The conformational change of BFM is considered to be similar
9
to that of 1,3-dinaphthylpropane (DNP) and dipyrenylalkanes
1
0−13
(
DPA)
in which the conformations in the ground and
excited states are relatively well-described. One major difference
among BFM, DNP, and DPA is that, while the part
chromophores in DNP and DPA undergo a torsional motion
It was shown in a previous study that the TD DFT B3LYP/cc-
pVTZ//B3LYP/cc-pVTZ calculations overestimate within only
0
0
6.8 ± 5.2% the 0 bands of 13 organic compounds containing at
2
273
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The Journal of Physical Chemistry A
Article
of the oscillator strengths was performed by using a Gaussian line
shape function having a unique fwhm of 0.05 eV. Note that the
fwhm of 0.05 eV is arbitrarily chosen and thus does not reveal a
physical meaning. We found that the broad and smooth sum
spectra as shown in Figures 3 and 4 reproduced by using the
value for the fwhm are similar to the experimental absorption
spectra in the threshold region from 280 to 310 nm. This can be
taken as an indication that the three conformations exist in the
solutions with almost equal propensity.
Figure 5 compares the absorption, fluorescence, and
fluorescence excitation spectra of the BFM and EFFM solutions.
Figure 3. Absorption spectrum of a 1.0 × 10−5 M bis(9-fluorenyl)-
methane solution in cyclohexane and comparison with the simulated
absorption spectra of various bis(9-fluorenyl)methane conformers
evaluated with the TD DFT B3LYP/cc-pVTZ//MP2/6-31G(d)
method. The vertical lines refer to the oscillator strengths.
Figure 5. Fluorescence excitation spectrum of a 1.0 × 10− M bis(9-
fluorenyl)methane (BFM) solution in cyclohexane is compared with the
absorption spectrum, and fluorescence spectrum of 1.2 × 10− M 9-(9-
ethylfluorenyl)-9′-fluorenylmethane (EFFM) solution in cyclohexane is
compared with that of the BFM solution. (a): Fluorescence excitation
spectrum of BFM. (b): Absorption and fluorescence spectra of BFM.
5
4
(c): Absorption and fluorescence spectra of EFFM.
A previous report on BFM indicates that the weak and violet-
emission band is responsible for the fluorescence from the
4
intramolecular excimer. Figure 5a compares the intensities for
the fluorescence excitation spectra measured by monitoring the
several fluorescence emitting 315 nm (monomeric fluorescene),
3
4
45 nm (mixture of monomeric and excimer fluorescence), and
05 nm (excimer fluorescence) photons. The three spectra reveal
similar peak shapes, and the intensity ratio for I (λmon = 315 nm)/
I (λmon = 405 nm) is measured to be 16.1 ± 2.7. Interestingly, the
value is very close to the intensity ratio for I (monomeric
fluorescence)/I (excimer fluorescence) as can be seen in the
fluorescence spectrum measured at λexc = 300 nm (Figure 5b).
The observation of the photophysical occurrences can be taken
as an indication that any highly excited states rapidly decay to the
lowest excited states, equilibrium among which may then be
attained. Afterward, the first excited-state monomer and excimer
emit monomeric and excimer fluorescene, respectively.
Figure 4. Absorption spectrum of a 1.2 × 10−4 M 9-(9-ethylfluorenyl)-
9
′-fluorenylmethane solution in cyclohexane and comparison with the
simulated absorption spectra of various 9-(9-ethylfluorenyl)-9′-
fluorenylmethane conformers evaluated with the TD DFT B3LYP/6-
3
11+G(d)//MP2/6-31G method. The vertical lines refer to the
oscillator strengths.
8
least one phenyl group. The rms deviation is shown to be as low
8
as 0.177 eV. Note that the simulated spectrum was shifted to red
for better comparison between theory and experiment by using a
Considering the structure of BFM, it is possible that there may
exist several conformers which are trapped in potential minima in
8
scaling factor of 0.93, the value used previously. Line spreading
2
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The Journal of Physical Chemistry A
Article
the electronic ground state. On the basis of MO calculations, the
fluorene excimer geometry has been proposed that two parallel
molecules are displaced along the short axis by about one
carbon−carbon bond length. In BFM, this geometry for excimer
configuration is not possible because of the interconnection
between two fluorenyl moieties through the central C atom. In an
attempt to enhance the violet-band intensity and to help identify
the band, as described above, we synthesized a structural analog
of BFM, EFFM. We presume that the steric hindrance arising
from the presence of the ethyl group will be able to enhance the
population of the sandwich conformer in the ground state and in
the excited state as well. As shown in Figure 5, the fluorescence
intensity of the electronically excited gg′ conformation of EFFM
is observed to be appreciably enhanced. In the pyrene crystal,
pairs of molecules align in a sandwich-like configuration with a
gg′ conformations. In our previous paper, the temperature
dependence of the excimer formation rate k was evaluated by
a
measuring τ at various temperatures. If the excimer dissociation
F
process is negligible, the excimer-formation rate constant can be
calculated from eq 1.
ka = 1 − (k + k ) ≈
1
τF
− kM
r
nr
τF
(1)
where 1/k and τ are the radiative and fluorescence lifetimes of
r
F
monomer molecules, respectively. k is the nonradiative decay
nr
rate constant excluding the excimer formation process. k may
M
be obtained from the lifetime of fluorene molecule itself, which
appears to be 10 ns in cyclohexane. Using the activation energy of
10 −1
1
5 kJ/mol and the Arrhenius factor of 5 × 10
s
measured
4
before in our laboratory, we can obtain the following equation:
3.4 Å interplanar separation in the ground state. Upon electronic
3
excitation, this separation becomes shorter via one-dimensional
motion of the two adjacent pyrene molecules toward each other.
The time scale has been measured to be 140 fs by Williams and
⎛ 1.9 × 10 ⎞
1
0 −1
k = 5 × 10 s exp⎜−
⎟
⎠
a
⎝
T
(2)
3
3
Nelson.
The energy barrier is close to that of other molecules forming
9
13
Kinetic parameters of the time evolution and decay of the
monomeric and excimer fluorescence are informative as to the
dynamics of the excimer formation and dissociation. When we
excited a BFM solution in pentane at 310 nm, we observed three
decay components of 0.5 ns (24%), 4.6 ns (68%), and 7.0 ns
excimers such as dinaphthylpropane and dipyrenylpropane.
The unusually low pre-exponential factor is also a characteristic
9
,13
feature corresponding to excimer formation processes.
At
room temperature, we evaluated the excimer-formation rate
7
−1
constant to be 8.5 × 10 s , which is low in comparison with the
8
−1
(
8%) leading to the 310 nm normal fluorescence. Figure 6
monomer-decay rate constant (1/τ ), 2.2 × 10 s , giving rise to
F
presents the fluorescence decay profile of the monomer of BFM
in n-pentane, n-octane, and hexadecane solutions by exciting at
the extremely low excimer fluorescence. The third component of
the monomeric decay has a low amplitude with a longer lifetime
(τ = 7 ns). This may be assignable to the delayed fluorescence
F
2
90 nm and by monitoring at 310 nm.
which has been frequently observed in other intramolecular
1
1,12
excimers.
The longer component of the excimer decay of
2
9.0 ns is a characteristic feature of the excimer decay.
The decay and rise time constants are also found to depend
upon the viscosity of the solvents probably because of frictional
resistance of the solvent molecules. The viscosity dependence of
the lifetimes of BFM is shown in Table 3.
The similar decay pattern was also observed in the octane and
hexadecane solutions even if we observed slightly different
kinetic parameters as shown in Table 3. In a rigid matrix
incapable of the free torsional motion along the C−C axis, how
do the time profiles of the monomeric and excimer decays
change? We elucidated kinetic behaviors of the monomeric and
excimer decays. As listed in Table 3, the decay components of the
monomeric fluorescence are close to those observed in the liquid
solutions in the present study, whereas those of the excimer
fluorescence are not. It is very intriguing not to observe the rising
components in the excimer decay in a PMMA matrix. We can
interpret it as being due to the existence of the perfect sandwich
conformer prior to the laser irradiation in the matrix.
A similar fluorescence decay pattern is observed in the time-
resolved fluorescence decay of EFFM. Figure 7 reveals the
fluorescence decay profile of the monomer of EFFM in n-
pentane, n-octane, and hexadecane solutions by exciting at 302
nm and by monitoring at 320 nm.
The fluorescence decay pattern of EFFM monitored at 320 nm
is contrasted with that of BFM. The overall decay appears to have
two components. The decay curve at 395 nm of EFFM is not
observed to have rise time as observed in the BFM system. This
may correspond to the direct electronic excitation of the
sandwichlike configuration already formed prior to laser
irradiation because the gg′ configuration in the electronic
ground state could be compared in the population with the
other orthogonal and tg′ configurations.
Figure 6. Fluorescence decay profiles of 1.0 × 10−5 M bis(9-
fluorenyl)methane solutions in various solvents. λexc = 290 nm, λmon
=
310 nm.
The two shorter decay components of 0.5 ns (24%) and 4.6 ns
68%) appear to correspond to the two rise components of 0.6 ns
11%) and 4.2 ns (32%) giving rise to the 390-nm excimer
(
(
fluorescence. The monomeric decay of 0.5 ns and the excimer
rise of 0.6 ns are presumably attributed to the transformation of
the near-sandwich conformer to the perfect face-to-face
conformation. This indicates that a near-sandwich conformer
close to the excimer conformation already exists in the ground
state. The excimer formation from this geometry is so fast that
the time scale is beyond the time resolution of TCSPC used in
this work. The ultrafast study of excimer formation in pyrene
10
molecular crystal is worthy of mention. We presume that the
medium component of the monomeric fluorescence in BFM of
4.6 ns and that of the excimer fluorescence of 4.2 ns are
attributable to the conformational change from the og or tg′ to
2
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The Journal of Physical Chemistry A
Article
Table 3. Viscosity Dependence of the Fluorescence Lifetimes
of Bis(9-fluorenyl)methane (BFM)
the time evolution and decay of the fluorescence are listed in
Table 4.
a
viscosity
(cP)
collection
relative
lifetimie
b
solvent
pentane
wavelength (nm)
amplitude
(ns)
Table 4. Viscosity Dependence of the Fluorescence Lifetimes
of 9-(9′-Ethylfluorenyl)-9-fluorenylmethane (EFFM)
a
0.22
310
0.24
0.5
4.6
7.0
0.6
4.2
29.0
1.4
5.3
0.7
4.9
29.5
1.3
5.6
1.9
5.7
29.4
0.5
0
0
.68
.08
viscosity
(cP)
collection
relative
lifetime
b
solvent
pentane
wavelength (nm)
amplitude
(ns)
3
90
−0.11
0.32
.57
0.20
.80
−0.14
0.31
.55
0.19
.81
−0.11
0.28
.61
0.27
0.22
320
0.13
0.3
1.0
0.3
2.8
3.7
0.8
2.3
0.6
17.3
0.1
3.0
0.2
2.9
33.6
0.9
−
0
.87
−0.01
0.48
.51
0.95
.05
−0.47
.53
0.27
.73
0
3
95
octane
0.55
3.60
∞
310
90
−
0
0
3
octane
0.55
3.60
320
95
320
95
−
0
0
3
hexadecane
310
90
0
0
hexadecane
3
0
−
3
0.01
0.06
0
−
PMMA
matrix
310
0
.93
PMMA
matrix
∞
320
0.17
0
0
.37
.36
3.6
6.9
0
0
.68
.15
5.0
8.1
3
90
0.50
0.6
0
0
.43
.07
2.6
3
95
0.42
.38
0.20
0.6
15.5
0
3.4
a
The excitation wavelength is 290 nm. Concentration = 1.0 × 10−5 M.
The lifetimes and amplitudes are within the errors of 0.2 ns and 10%,
23.4
b
a
The excitation wavelength is 302 nm. Concentration = 1.0 × 10−5 M.
The lifetimes and amplitudes are within the errors of 0.2 ns and 10%,
respectively.
b
respectively.
The time scales of 0.5 ns in n-pentane and 1.0 ns in other
higher viscous solvents are too long for barrierless rate processes.
One possibility is that there may be present a small barrier
comparable to the thermal energy, which is undetectable because
of the broad instrument function of our apparatus. The slow
process which we have attributed to the conformational change
of the og form to the excimer is dependent upon viscosity and
temperature. The viscosity dependence of the rate of the excimer
formation of BFM and EFFM is mild, from which we can deduce
that the conformational changes giving rise to the excimer
formation almost involve the torsional rotation around the fixed
C−C bonds.
Figure 7. Fluorescence decay profiles of 1.2 × 10−4 M 9-(9-
ethylfluorenyl)-9′-fluorenylmethane solutions in various solvents. λ
exc
=
302 nm, λ = 320 nm.
mon
It is surprising not to observe the longer component of the
excimer fluorescence in a EFFM solution in pentane. In contrast,
the longer component is observed in other solvent and matrix
media. We have also not observed any rising component of the
excimer fluorescence in an EFFM matrix in PMMA, presumably
because there exists the perfect face-to-face conformer before the
laser irradiation. Degree of freedom for the conformational
change in BFM is higher than that in 9,9′-bifluorene. The
dynamics are well-described by the conventional theory for
excimer formation. In contrast to 9,9-bifluorene, this study
showed that the barrier-crossing process of BFM in the 0.2−3.3
cP solvent viscosity falls into the diffusive regime in the Kramers’s
It is described above that the substitution of a 9-H atom in
BFM with an ethyl group influences the energies and the
corresponding population of the conformers in the ground and
excited states. Next, we chose to evaluate the kinetic parameters
associated with the interconversion of the conformers. The
fluorescence decay pattern of EFFM obtained by exciting at 302
nm and monitoring at 320 nm is contrasted with that of BFM.
The two rapid decay components of 0.3 and 1.0 ns of the
monomeric fluorescence almost correspond to the rise
components of the excimer fluorescence. To examine the
viscosity dependence of the rate of formation and dissociation
processes, we measured the lifetimes of EFFM monomers and
excimers in several alkane solvents (n-octane, n-tetradecane, and
n-hexadecane) and a PMMA matrix. The kinetic parameters for
34
theory. The conformational change is an activated rate process
with the activation energy of 15 kJ/mol and the viscosity range
(0.2−3.3 cP) lies in the diffusive regime in the Kramers’s theory.
2
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The Journal of Physical Chemistry A
Article
CONCLUSIONS
We searched the three conformations in both BFM and EFFM.
The single-point energy calculation on BFM with the MP2/6-
Professor Aquilanti of Department of Chemistry, University of
Perugia, Perugia, Italy, who made it possible to work comfortably
at the department for six months and provided useful
information for the quantum calculations.
■
3
11++G(d,p)//MP2P/6-31G(d) method reveals that the gg′
geometries are achieved as the lowest conformers in the diaryl
compounds. We simulated the absorption spectra of each
conformer of BFM and EFFM with TD DFT calculations. The
excimer fluorescence intensity of EFFM is observed to be
appreciably enhanced in comparison with that of BFM. When a
BFM solution in pentane was excited at 310 nm, the first two
decay components of 0.5 ns (24%) and 4.6 ns (68%) leading to
the 310-nm monmeric fluorescence exactly correspond to the
two rise components of 0.6 ns (11%) and 4.2 ns (32%) giving rise
to the 390 nm excimer fluorescence. The shorter component of
the monomeric decay of 0.5 ns and the excimer rise of 0.6 ns are
presumably attributed to the transformation of the near-
sandwich conformer to the perfect face-to-face conformation.
The medium component may be attributed to the conversion of
the og and tg′ conformers to the perfect gg′ conformer. The
longer component of 29.0 ns in the decay of the 390 nm
fluorescence is a characteristic feature of the excimer decay. We
have not observed the rising components in the excimer decay in
matrices of BFM and EFFM in PMMA probably because of the
existence of the perfect sandwich conformer prior to the laser
irradiation. The dynamics are well-described by the conventional
theory for excimer formation. In contrast to 9,9-bifluorene, this
study showed that the barrier-crossing process of BFM in the
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ASSOCIATED CONTENT
Supporting Information
R2PI-TOF mass and R2PI spectra of the decomposition
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charge via the Internet at http://pubs.acs.org.
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Corresponding Author
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NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was published ASAP on March 5, 2014, with an
incorrect version of the Supporting Information file. The
corrected version was published ASAP on March 14, 2014.
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