Quantitative evaluation of thermodynamic parameters for thermal back-reaction after mechanically induced fluorescence change

Article abstract of DOI:10.1039/c3ra43321h

Kinetics of the thermal back-reaction of β-diketonate boron difluoride complexes after mechanical perturbation were evaluated by fluorescence intensity changes for the first time, suggesting that the activation parameters of the reaction intermediates gov

Full text of DOI:10.1039/c3ra43321h

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Quantitative evaluation of thermodynamic parameters
for thermal back-reaction after mechanically induced
fluorescence change†
Cite this: DOI: 10.1039/c3ra43321h
Received 1st July 2013
Fuyuki Ito* and Takehiro Sagawa
Accepted 16th August 2013
DOI: 10.1039/c3ra43321h
www.rsc.org/advances
Kinetics of the thermal back-reaction of b-diketonate boron difluor- most probably originate from changes in intermolecular inter-
ide complexes after mechanical perturbation were evaluated by actions induced by intermolecular distance changes.
fluorescence intensity changes for the first time, suggesting that the
The b-diketonate boron diuoride complex exhibits a uo-
activation parameters of the reaction intermediates governed inter- rescence spectral change by mechanical perturbation depend-
8–11
molecular interactions such as hydrogen bonding assisted by ing on its concentration.
Fraser et al. reported a reversible
substituent groups.
mechanochromic uorescent organic solid based on the 4-tert-
butyl-4 -methoxydibenzoylmethane (trade name: avobenzone)
0
boron diuoride complex, which shows different emission
depending on the crystal type. The emission color signicantly
Organic uorescent solids have been used widely in lumines-
cent materials such as electroluminescent (EL) devices.
12
1
red-shis upon rubbing/smearing the samples. The samples
then revert back to the original emission species over time. This
is the rst nding of a thermal-back type (t-type) uorescent
mechanochromism phenomenon. However, the kinetic and
thermodynamic properties of t-type mechanochromic
compounds have not been totally claried. In this communi-
cation, we studied and quantitatively evaluated for the rst time
the thermodynamic parameters for thermal back-reaction aer
mechanically induced uorescence change.
The molecular structure of boron diuoride b-diketonate
complexes is shown in Chart 1. In this study we used 1-(4-tert-
butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione (ab), 1,3-
bis-(4-methoxyphenyl)-1,3-propanedione (2a), and 1,3-bis-(4-
tert-butylphenyl)-1,3-propanedione (2b) as ligands. The boro-
Organic molecular solids or molecular assemblies are typically
based on van der Waals interactions such as p–p interactions or
hydrogen bonding interactions. These interactions promote the
development of novel functions and properties, which differ
from those in the monomer state. In the development of
luminescent materials based on organic molecules, it is
important to understand the properties and functions, not only
for the monomer species, but also for the assembled species. In
photochromism in a molecular crystal system, for example,
cooperative interaction results in a change of the crystal struc-
ture by the chromic reaction. Kobatake and Irie et al. report that
molecular crystals based on diarylethene chromophores with
sizes ranging from 10 to 100 mm exhibit rapid and reversible
macroscopic changes in shape and size induced by UV and
3 2 2 2
nation was performed by BF $OEt in CH Cl according to a
2
visible light. The densely packed crystals were proven to be
12
previous report. The purication was carried out with column
useful to link molecular-scale events to the macroscale move-
ment of materials, which leads to mechanical perturbation. In
contrast, it is believed that mechanical forces can perturb
intermolecular interactions such as molecular distance. Over
the past decade, mechano- and piezo-chromic luminescent
3,4
effects on organic solids have been reported, especially on
5–7

uorescent materials.
The mechanochromic phenomena
Department of Chemistry, Faculty of Education, Shinshu University, 6-ro,
Nishinagano, Nagano 380-8544, Japan. E-mail: to@shinshu-u.ac.jp; Fax: +81-26-
2
38-4114; Tel: +81-26-238-4114
†
Electronic supplementary information (ESI) available: XRD spectra,
uorescence spectra and Arrhenius and Eyring plots of 2aBF
2 2
and 2bBF , IR
spectra of abBF . See DOI: 10.1039/c3ra43321h
2
Chart 1 Molecular structure of boron difluoride b-diketonate complexes.
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ꢁ
3
ꢁ3
chromatography. A 2.0 ꢀ 10
mol dm
dichloromethane
solution was dropped on paraffin-coated weighing paper, and
then the solvent-evaporated sample was used for spectroscopic
studies. Fluorescence spectra were recorded on a Shimadzu RF-
5300PC uorescence spectrophotometer. In order to apply a
mechanical perturbation to the boron diuoride b-diketonate
complexes, the samples were rubbed with a spatula. The
temperature around the sample was controlled by a home-made
system using a rubber heater (Hakko) and digital temperature
controller (Omron E5CN-QT).
Fig. 1 shows uorescence spectra of abBF on paraffin-coated
2
weighing paper at 303 K. The uorescence spectra were
normalized to the maximum intensity. The drop-casted abBF
2
powder exhibited a blue emission that peaked at 460 nm as
shown in blue in Fig. 1. The uorescence spectrum originated
12
from a single crystal or dendritic solid as previously observed.
Aer rubbing with a spatula, a new uorescence band appeared
around 500 nm with a shoulder at 550 nm, originating from the
12
Fig. 2 Changes in fluorescence intensity of abBF
2
monitored at 550 nm as a
amorphous state of abBF2. The sample aer rubbing emitted a
yellow-green color under the UV lamp. The uorescence inten-
sity up to 530 nm decreased as time elapsed. Aer 1030 min, the
function of time after mechanical perturbation. The temperature is (a) 296, (b)
03, and (c) 313 K. The solid lines indicate the best-fitting curves based on a
3
double-exponential decay function.

uorescence spectrum showed peaks around 460 and 500 nm,
the emissive color appearing green under the UV lamp. These
observations indicated that the yellow uorescent species was
changed to green at room temperature, demonstrating abBF
2
as rescence decay
Table 1 Rate constant of thermal back-reaction for abBF
2
from fitting of fluo-
a
a t-type mechanochromic molecule.
ꢁ1
Rate constant/s
In order to conduct a quantitative kinetic analysis for
thermal back-reaction aer mechanical perturbation, we
measured the uorescence intensity change. Fig. 2 shows
changes in uorescence intensity at 550 nm excited at 370 nm
as a function of time. The temperature was maintained during
the measurement at (a) 296, (b) 303, and (c) 313 K. The uo-
rescence intensity at 550 nm decreased with elapsed time,
which can be reproduced with a double-exponential decay
function obeying rst-order kinetics. The rate constants of the
faster (k ) and slower (k ) components obtained by least-squares
Temperature/K
k
F
k
S
ꢁ
ꢁ
ꢁ
3
2
2
ꢁ4
ꢁ4
ꢁ4
ꢁ4
ꢁ3
2
3
3
3
3
96
03
13
23
33
8.06 ꢀ 10
1.70 ꢀ 10
2.25 ꢀ 10
3.05 ꢀ 10
8.52 ꢀ 10
5.03 ꢀ 10
6.43 ꢀ 10
7.83 ꢀ 10
9.72 ꢀ 10
1.95 ꢀ 10
ꢁ2
ꢁ2
a
F S
The uorescence intensity at 550 nm excited at 370 nm. The k and k
correspond to faster and slower rate constants.
F
S

tting are listed in Table 1. The two rate constants increase with
increasing temperature. The changes in uorescence intensity
quantitatively exhibit exponential decay from the manual values exhibited variations which were within the expected
smearing. The mechanical perturbation using a spatula has an experimental error. Therefore, no quantitative analysis on the
inherently low reproducibility. It is therefore also true that the ratio of the pre-exponential factor for the two decay components
reproducibility of the initial uorescence intensity and the was performed.
absolute values of the pre-exponential factor were relatively low.
The temperature dependence of the rate constants allows us
In this paper, however, we focus on the kinetic parameters to determine the activation parameters of the amorphous–
associated with changes in uorescence intensity, and these crystal phase transition of the boron diuoride b-diketonate
complexes. The kinetic law of reaction k ¼ A exp(ꢁE /RT) can be
a
evaluated and used to provide the activation parameters, where
A, E
gas constant, and temperature, respectively. Fig. 3a shows the
Arrhenius plot of the thermal back-reaction of abBF together
with a straight line by least-squares tting. The A and E
a
, R, and T are the pre-exponential factor, activation energy,
2
a
were
, and
6
ꢁ1
ꢁ1
estimated to be 1.05 ꢀ 10 s and 45.8 kJ mol for k
F
1
ꢁ1
ꢁ1
2
.97 ꢀ 10
s
and 27.2 kJ mol
for k , respectively. The
S
‡
‡
enthalpy and entropy of activation energies (DH and DS ) for
the thermal back-reaction of abBF were also estimated from the
2
Eyring plot as shown in Fig. 3b, using the equation k ¼ (k
B
T/h)
2
Fig. 1 Fluorescence spectra of abBF at 303 K. The excitation wavelength is 370
‡
‡
nm.
exp(DS /R)exp(ꢁDH /RT), where k
is the Boltzmann constant.
B
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‡
substituents, respectively. The DG values at 303 K were esti-
ꢁ
1
mated to be 77.2 kJ mol for the faster component and 83.3 kJ
ꢁ
1
ꢁ1
mol for the slower component for 2aBF , and 78.8 kJ mol
2
ꢁ
1
for the faster component and 84.3 kJ mol for the slower
component for 2bBF . It can be seen from comparing these
values with abBF that DG is not dependent on the substituent
2
‡
2
group. These ndings indicate that the substituent group of the
boron diuoride b-diketonate complexes does not affect the
energy barrier for the system, but does affect the thermal back-
2
Fig. 3 Arrhenius (a) and Eyring (b) plots for thermal back-reaction of abBF .
Closed and open circles correspond to faster and slower components.
reaction rates. The formation process of the activated complex
‡
(transition state) is governed by the entropic term. The DH
‡
ꢁ1
value is much larger than the formation energy by the van der
The resulting DH s are 43.2 kJ mol for the faster component
ꢁ
1
ꢁ1
‡
Waals interaction (generally 1 kJ mol ), but is comparable
to the dipolar interaction and hydrogen bonding interaction
and 24.6 kJ mol for the slower component, and the DS s are
ꢁ
1
ꢁ1
ꢁ
104 J K for the faster component and ꢁ191 J K for the
ꢁ
1
‡
(10–150 kJ mol ).
slower component. The estimated DS values are negative,
suggesting the order parameter of the activated complex is
higher than that of the initial amorphous phase. The thermo-
dynamic ndings conrmed that the thermal back-reaction of
Fraser et al. suggested that the multiple interactions, such as
strong dipolar nature of the molecules, arene stacking and
C(arene)–H/F hydrogen bonds, may be the driving force for the
aggregation of the boron diuoride b-diketonate complexes
2
abBF originates from the amorphous–crystal transformation.
We think that the amorphous–crystal transformation correlates
with the crystal growth process from the melt states. The DS for
the faster component is lower than that for the slower compo-
nent. The free energy barriers (DG ) of abBF at 303 K for faster
and slower components were 74.7 kJ mol and 82.5 kJ mol
respectively. These ndings indicate that the reaction via the
lower-order activation complexes (transition state) is the
dominant pathway for thermal back-reaction aer mechanical
perturbation.
In order to determine the kinetic and activation parameters
2
for the thermal back-reaction of abBF , we examined the
substituent dependence for the reaction. Compound abBF has
tert-butyl and methoxy substituents [the ligand is 1-(4-tert-
butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione]. Thus, we
studied 1,3-bis-(4-methoxyphenyl)-1,3-propanedione (2a) and
,3-bis-(4-tert-butylphenyl)-1,3-propanedione (2b) as ligands
next. Both 2aBF and 2bBF also showed a crystal-to-amorphous
10–13
upon thermal treatment or recovery aer smearing.
Because
‡
the kinetic mode related to the methoxy substituent preferen-
tially promotes crystal reformation over the tert-butyl substit-
uent, the driving force and transition state for the crystallization
‡
2
ꢁ
1
ꢁ1
of abBF
action, but also to the C(arene)–H/O(methoxy) interaction.
Preliminary studies of the IR spectra of abBF indicate that the
2
will contribute not only to the C(arene)–H/F inter-
,
14
2
1
ꢁ
bands attributed to C–H stretching (850 cm ) and out-of-plane
ꢁ1
bending modes (1560–1580 cm ) were dramatically changed
before and aer the smearing process (see ESI†). Thus, it is
found that intermolecular interactions, especially the hydrogen
bonding interaction, inuence the mechanouorochromism
for the boron diuoride b-diketonate complexes in terms of
kinetic and thermodynamic parameters.
2
In summary, we have studied and evaluated quantitatively
the thermodynamic parameters for the thermal back-reaction of
1
abBF aer a mechanically induced uorescence change. The
2
2
2
changes in uorescence intensity exhibited exponential decay
aer the mechanical perturbation that depended on tempera-
ture. The activation parameters of the amorphous–crystal phase
transition suggested that the transition state to the activated
complex was higher than that of the initial amorphous phase
phase change subsequent to the mechanical perturbation, as
conrmed by XRD spectra, and the substituent dependence of
the rate constants and the activation parameters were deter-
mined in the same manner. These experimental data are
included in the ESI†. The activation parameters are listed in
Table 2. The activation parameters of the faster and slower
‡
based on the negative DS values. The substituent groups
inuenced the thermal back-reaction rates, which most prob-
ably controlled intermolecular interactions such as hydrogen
bonding between the boron diuoride b-diketonate complexes.
These ndings will guide the design of mechanouorochromic
reactions by controlling the substituent groups, and also
inuence studies on the kinetics and thermodynamics of the
crystal nuclei formation process from a melted state to a crystal
state. Detailed studies on the substitution dependence of the
thermal back-reaction by thermal analysis (DSC) of the crystals
and IR spectral changes are now in progress.
components of abBF
nent of 2aBF and the faster component of 2bBF
This indicates that the kinetic modes of the faster and slower
components in abBF are related to the methoxy and tert-butyl
2
are similar to that of the faster compo-
2
2
, respectively.
2
Table 2 Kinetic and activation parameters of the thermal back-reaction
‡
‡
E
a
A
s
DH
DS
ꢁ1
ꢁ1
ꢁ1
ꢁ1
Compound Component kJ mol
kJ mol
J K
6
1
5
2
1
ꢁ
abBF
2
k
k
k
k
k
k
F
S
F
S
F
S
45.8
27.2
44.1
33.6
23.1
17.1
1.05 ꢀ 10
2.97 ꢀ 10
2.36 ꢀ 10
2.82 ꢀ 10
2.65 ꢀ 10
2.70 ꢀ 10
43.2
24.6
41.6
31.1
20.5
14.5
ꢁ104
ꢁ191
ꢁ116
ꢁ172
The authors thank Dr Kennosuke Itoh (Shinshu University)
for the NMR measurements, and Shimadzu corporation for use
of MIRacle 10 (ATR attachment). This work was partly sup-
2
2
aBF
2
bBF
2
ꢁ192 ported by Nanotechnology Platform Program (Molecule and
1
ꢁ230 Material Synthesis) of the MEXT, Japan.
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