Unveiling the triplet state of a 4- amino-7-Nitrobenzofurazan Derivative in
cyclohexane
Christopher Sveen1, Nicolas Macia1, Vanina Zaremberg2, Belinda Heyne*1
1Chemistry
Department, University of Calgary, 2500 University Drive N.W. Calgary, Alberta,
T2N 1N4, Canada
2
Department of Biological Sciences, University of Calgary, 2500 University Drive N.W.
Calgary, Alberta, T2N 1N4, Canada
Email: bjmheyne@ucalgary.ca
1.
Kinetics traces of NBD-hexane in deaerated cyclohexane, fitting
analysis, and confirmation of a second order kinetics
S2
2.
Transient spectrum of NBD-hexane in deaerated cyclohexane in
presence of naphthalene.
S8
3.
Confirmation of triplet state by quenching experiment with
anthracene.
S9
4.
Cyclic voltammogram of NBD-hexane.
S11
5.
Effect of NBD-hexane concentration on kinetic of the triplet state.
S13
6.
Photobleaching of NBD-hexane in cyclohexane under visible light in
the presence of oxygen.
S14
7.
Triplet-triplet transient spectrum of NBD-hexane in deaerated
ethanol.
S15
S1
1. Kinetics traces of NBD-hexane in deaerated cyclohexane and fitting
analysis
Under our experimental conditions, the kinetics obtained at 360 nm, 435 nm and
640 nm could not be fitted via a simple mono-exponential decay, indicating a more
complex mechanism of deactivation of the triplet state. Our data were best fitted by
the combination of a first plus a second order kinetics, associated to the deactivation
of the transient by the following two simultaneous non-interacting decay processes:
1
T ¾k¾
® M (eq. 1)
2
T +T ¾k¾
® M + M (eq. 2);
where T represent the transient species, k1 is the rate constant for the first order
reaction having units of s-1, k2 is the rate constant for the second order reaction
expressed in M-1 s-1, and M represents a molecule in the ground state. The rate law
associated to these parallel reactions is given by:
d[T ]
2
= -k1 [T ] - 2k2 [T ] , (eq. 3)
dt
where [T] represents the concentration in transient species. This differential
equation can be solved, affording the following expression for the concentration in
transient species:(1)
[T ] =
[T ] 0 e-k t
1
ì æk ö
ü
1+ í2 ç 2 ÷ [T ] 0 (1- e-k1t )ý
î è k1 ø
þ
, (eq. 4)
where [T]0 is the initial concentration in transient species after laser pulse. This
expression can be rewritten in term of variation of absorbance as:
S2
DA0 e-k1t
, (eq. 5)
DA =
ì æk ö
ü
1+ í2 ç 2 ÷ [T ] 0 (1- e-k1t )ý
î è k1 ø
þ
where A represents the variation in absorbance, the subscript 0 refers to the initial
time.
a) Kinetic at 360 nm
a)
b)
0.003
0.03
0.002
Residuals for fit at 360 nm
DAbsorbance at 360 nm
0.025
0.02
0.015
0.001
0
-0.001
-0.002
-0.003
20
0.01
30
40
50
60
70
80
90
100
Time (ms)
0.005
0
0
20
40
60
80
100
Time (ms)
Figure S1: a) Kinetic obtained at 360 nm after excitation with a pulsed laser at 425 nm of a
41 M NBD-hexane in deaerated cyclohexane with an overlay in red of the curve fitting
according to equation 5. b) Residuals for the curve fitting according to equation 5.
S3
b) Kinetic at 435 nm
a)
b)
0.0015
0
0.001
residuals for fit at 435 nm
DAbsorbance at 435 nm
-0.01
-0.02
0.0005
0
-0.0005
-0.03
-0.001
-0.0015
-0.04
20
30
40
50
60
70
80
90
100
Time (ms)
-0.05
-0.06
-0.07
0
20
40
60
80
100
Time (ms)
Figure S2: a) Kinetic obtained at 435 nm after excitation with a pulsed laser at 425 nm of a
41 M NBD-hexane in deaerated cyclohexane with an overlay in red of the curve fitting
according to equation 5. b) Residuals for the curve fitting according to equation 5.
S4
c) Kinetic at 640 nm
a)
b)
0.0015
0.014
0.001
Residuals for fit at 640 nm
0.012
DAbsorbance at 640 nm
0.01
0.0005
0
-0.0005
0.008
-0.001
0.006
-0.0015
20
30
40
50
60
70
80
90
100
Time (ms)
0.004
0.002
0
0
20
40
60
80
100
Time (ms)
Figure S3: a) Kinetic obtained at 640 nm after excitation with a pulsed laser at 425 nm of a
41 M NBD-hexane in deaerated cyclohexane with an overlay in red of the curve fitting
according to equation 5. b) Residuals for the curve fitting according to equation 5.
S5
d) Summary of the curve fitting data
(nm)
360
435
640
A0
k1(s)-1
(SD)*
(SD)*
0.0262
0.066
(0.0003)
(0.004)
0.0556
0.069
(0.0002)
(0.001)
0.0134
0.058
(0.0001)
(0.003)
1(s)
k2’(s)-1
(SD)*
15.15
0.035
(0.04)
14.49
0.033
(0.001)
17.24
0.029
(0.003)
Table S1: *Data obtained from fitting the curves according to equation 5. 1 is the lifetime
extracted from the rate constant obtained for the first order decay, and k’2 expressed in
(s)-1 corresponds to the product of k2 (M-1 s-1) by [T]0 (M) in equation 5.
According to our data, the lifetime associated to the first order decay is very
similar for the three wavelengths probed, which is around 15 s. The occurrence of
a second order kinetic was confirmed by conducting an experiment where the
kinetic of a NBD-hexane solution in deaerated cyclohexane was monitored at
different laser energy. Varying the laser energy was achieved via the use of neutral
density filters and resulted in a change of the transient species concentration.(2) As
shown in Figure S4, a decrease in laser energy is associated to a decrease in the
S6
apparent rate of deactivation, which translates in an increase in the apparent halflife of the transient species.
a)
b)
12
27 mJ
18 mJ
10 mJ
0.03
0.025
Apparent half life at 360 (ms)
DAbsorbance at 360 nm
0.035
0.02
0.015
0.01
0.005
0
0
20
40
60
Time (ms)
80
100
11
10
9
8
7
6
5
10
15
20
Laser energy (mJ)
25
30
Figure S4: a) Kinetics obtained at 360 nm for a 41 M NBD-hexane solution in
deaerated cyclohexane after excitation with a pulsed laser at 425 nm with various
energies. b) Apparent half-life extracted from the kinetics in function of the laser
energy.
As it will be discussed later in this study, the transient species observed
corresponds to the triplet state of NBD-hexane. The second order kinetic is thus
associated to a triplet-triplet annihilation process.
S7
2. Transient spectrum of NBD-hexane in deaerated cyclohexane in presence of
naphthalene.
0.01
DAbsorbance
0.005
0
-0.005
-0.01
-0.015
350
400
450
500
550
600
650
700
Wavelength (nm)
Figure S5: Transient absorption spectrum of NBD-hexane (41 M) in deaerated
cyclohexane in the presence of 5.6 mM naphthalene obtained 7s (red), 19s (blue) and
34s (green) after laser excitation at 425 nm.
No change in the transient absorption spectrum of NBD-hexane could be
detected in the presence of naphthalene.
S8
3. Confirmation of triplet state by quenching experiment with anthracene.
In order to assign the multiplicity of the transient species observed in deaerated
cyclohexane, a triplet-triplet energy transfer experiment was performed using
anthracene as a triplet acceptor.(2, 3) Anthracene is an ideal candidate, as its ground
state presents no absorbance at the laser excitation wavelength of 425 nm (Figure
S6), and it possesses a low lying triplet state (ET= 178 kJ mol-1),(4) whose maximum
absorption is located at 420 nm.(5) As shown in figure S7, addition of athracene to a
deaerated solution of NBD-hexane in cyclohexane leads to the quenching of the
NBD-hexane kinetic at 640 nm, which is accompanied by a growth of the
characteristic band of the anthracene triplet state.
0.5
Absorbance
0.4
0.3
0.2
0.1
0
250
300
350
400
450
Wavelength (nm)
Figure S6: Absorption spectrum of a 4.3x10-5 M in cyclohexane.
S9
b)
0.01
0.08
0.008
0.06
DAbsorbance at 420 nm
DAbsorbance at 640 nm
a)
0.006
0.004
0.002
0.04
0.02
0
-0.02
0
0
50
100
150
Time (ms)
200
250
0
50
100
150
Time (ms)
200
250
Figure S7: Kinetics obtained after excitation with a pulsed laser at 425 nm of a 30
M NBD-hexane in deaerated cyclohexane in the absence (black) and in the
presence of anthracene (43 M, red): a) at 640 nm and b) at 420 nm.
S10
4. Cyclic voltammogram of NBD-hexane.
The cyclic voltammogram (CV) of NBD-hexane could not be recorded in
cyclohexane due to the extremely low solubility of the electrolyte necessary for this
experiment. For this reason, the CV was performed in acetonitrile.
The cyclic voltammogram was carried out in an all glass, thermo-jacketed,
three-electrode cell. Ag|AgCl|KCl3M was used as the reference electrode, which was
isolated from the working solution through a Vycor tip. A Pt wire was used as a
counter electrode and a glassy carbon electrode was used as the working electrode.
-Q water. The Pt wire counter
electrode was heated with a butane flame for 30 s prior to each measurement and
the
acetonitrile
electrolyte
contained
100
mM
of
tetrabutylamonium
hexafluorophosphate (Bu4NPF6). The cell was thermo-regulated using a circulating
water bath (Lauda, Ecoline Refrigerating Circulators RE-200 Series) set at 25 °C.
Argon was bubbled for 20 minutes preceding the CV, followed by a continuous Ar
blanket above the electrolyte during the entire electrochemical experiment. The
cyclic voltammetry experiments were conducted using an Autolab PG302
potentiostat and iR compensation was applied manually to each CV experiment. A
PC using FRA and GPES version 4.9 software controlled the electrochemical system
and was used for data analysis.
S11
0.0002
Aryl amine oxidation
0.00015
Current (A)
0.0001
5 10-5
0
-5
-5 10
-0.0001
Nitro reduction
-0.00015
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Potential (V)
Figure S8: Cyclic voltammogram of a 7.5 mM NBD-hexane solution in acetonitrile in the
presence of 100 mM Bu4NPF6.
As expected from NBD hexane, the CV displays a reversible reduction of the
nitro group, with what appears to be a reversible oxidation, corresponding of the
amine consistent with literature.
S12
5. Effect of NBD-hexane concentration on the kinetic of the triplet state.
In order to assess the possibility of an interaction between the triplet excited
state and the ground state of NBD-hexane, kinetics were recorded for different NBDhexane concentrations. As shown in Figure S9, a faster apparent kinetic is obtained
upon increasing NBD-hexane concentration while keeping the laser energy constant
(12 mJ), supporting thus quenching of the triplet state by ground state NBDhexane molecules.
a)
b)
Dabsorbance at 435 nm
0
-0.01
-0.02
-0.03
-0.04
0
50
100
150
Time (ms)
200
250
Figure S9: Kinetics obtained at 435 nm after excitation with a pulsed laser at 425
nm of a 30 M (black) and a 60 M (red) NBD-hexane in deaerated cyclohexane: a)
raw data and b) normalized kinetics in order to emphasize the different rates.
S13
6. Photobleaching of NBD-hexane in cyclohexane under visible light in the
presence of oxygen.
1.2
1
Absorbance
0.8
0.6
0.4
0.2
0
200
250
300
350
400
450
500
550
600
Wavelength (nm)
Figure S10: Photodegradation of NBD-hexane (41 M) in cyclohexane in the presence
of oxygen. Irradiations were performed under visible light in Luzchem photoreactor for
0 min (circle), 30 min (square), 60 min (diamond), 90 min (triangle up), 120 min
(triangle down), and 220 min (cross).
The increase in absorption in function of the irradiation time is due to
evaporation of the solvent and therefore concentration of the sample with
exposure time.
S14
7. Triplet-triplet transient spectrum of NBD-hexane in deaerated ethanol.
Figure S11: Transient absorption spectrum of NBD-hexane (34 μM) in deaerated
ethanol upon laser excitation at 464 nm.
References:
1. Capellos, C. and B. H. J. Bielski (1972) Kinetic Systems. Mathematical Description
of Chemical Kinetics in Solution. Wiley-Interscience, New York.
2. Cosa, G. and J. C. Scaiano (2004) Laser techniques in the study of drug
photochemistry. Photochem. Photobiol. 80, 159-174.
3. Sortino, S. and J. C. Scaiano (1999) Laser flash photolysis of tolmetin: a
photoadiabatic decarboxylation with a triplet carbanion as the key intermediate in
the photodecomposition. Photochem. Photobiol. 69, 167-172.
4. Montalti, M., A. Credi, L. Prodi, M. T. Gandolfi and Editors (2006) Handbook of
Photochemistry - Third Edition. CRC Press LLC.
5. Bensasson, R. and E. J. Land (1971) Triplet-triplet extinction coefficients via
energy transfer. Trans. Faraday Soc. 67, 1904-15.
S15