Nonradiative deactivation of singlet oxygen (1O2) by cubane and its derivatives

Article abstract of DOI:10.1021/ol802113j

(Chemical Equation Presented) The bimolecular quenching rate constants of singlet oxygen (¹O₂) by cubane and cubane derivatives were determined and found to be in the order of 10³-10⁴ M ^-1 s^-1. These values represent larger values than expected for aliphatic alkanes as a model for C-H vibrational deactivation. This result is explained by the occurrence of two different deactivation mechanisms: energy transfer to cubane C-H vibrational modes and the formation of a charge-transfer complex between ¹O₂ and cubane (¹O ₂^?- ...cubane^?+).

Full text of DOI:10.1021/ol802113j

ORGANIC
LETTERS
2
008
Vol. 10, No. 24
509-5512
Nonradiative Deactivation of Singlet
1
5
Oxygen ( O ) by Cubane and Its
2
Derivatives
†
Jeffrey R. Lancaster, Angel A. Mart ´ı , Juan L o´ pez-Gejo, Steffen Jockusch,
‡
§
Naphtali O’Connor, and Nicholas J. Turro*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
njt3@columbia.edu
Received September 9, 2008
ABSTRACT
1
The bimolecular quenching rate constants of singlet oxygen ( O ) by cubane and cubane derivatives were determined and found to be in the
2
order of 10 -10 M s-1. These values represent larger values than expected for aliphatic alkanes as a model for C-H vibrational deactivation.
3
4
-1
This result is explained by the occurrence of two different deactivation mechanisms: energy transfer to cubane C-H vibrational modes and
1
1
•-
•+
2 2
the formation of a charge-transfer complex between O and cubane ( O ···cubane ).
After its initial synthesis in 1964, it was clear that
diative deactivation of an excited-state species. We report
the quenching rate constants of singlet oxygen by cubane
and several of its derivatives (Scheme 1).
2
,5 3,8 4,7
pentacyclo[4.2.0.0 .0 .0 ]octane, also termed cubane,
might find numerous uses because of its novel geometric
1
structure. To date, cubane and its derivatives have shown
3
3
2
Molecular oxygen in its ground state ( Σ, O ) can be
2
potential for application as high energy fuels and explosives;
1
1
2
electronically excited to produce singlet oxygen ( ∆, O ).
as new novel materials, synthetic intermediates, and poly-
This excited-state decays inefficiently to the ground-state
since the transition is spin-, symmetry-, and parity-forbidden.
Despite this inefficiency, energy exchange by coupling with
oscillators of neighboring molecules (especially C-H and
O-H vibrations) has proven to be an important mechanism
3
mers; and as high refractive index fluids for immersion
4
lithography due to high electron densities. Here, the cubane
skeleton is used as an example of a highly strained
hydrocarbon backbone with which to examine the nonra-
1
5
†
for the nonradiative relaxation of O
2
. The study of the
Current address: Department of Chemistry, Rice University, Houston,
TX 77005.
1
quenching of O
2
in different solvents has shown that
‡
Current address: Departamento de Qu ´ı mica Org a´ nica, Universidad
different functional groups quench singlet oxygen with
Complutense de Madrid, Madrid, Spain 28026.
§
Current address: Department of Chemistry, Lehman College, City
University of New York, Bronx, NY 10468.
(
1) Eaton, P. E.; Cole, T. W. J. Am. Chem. Soc. 1964, 86, 962.
(4) Zimmerman, P. A.; Byers, J.; Rice, B.; Ober, C. K.; Giannelis, E. P.;
Rodriguez, R.; Wang, D.; O’Connor, N.; Lei, X.; Turro, N. J.; Liberman,
V.; Palmacci, S.; Rothschild, M.; Lafferty, N.; Smith, B. W. Proc. SPIE
2008, 6923, 69230A.
(
2) (a) Zhang, M.-X.; Eaton, P. E.; Gilaridi, R. Angew. Chem., Int. Ed.
2
2
000, 39, 401. (b) Gejji, S. P.; Patil, U. N.; Dhumal, N. R. THEOCHEM
004, 681, 117. (c) Schmidt, M. W.; Gordon, M. S.; Boatz, J. A. Int. J.
Quant. Chem. 2000, 76, 434.
3) (a) Eaton, P. E.; Pramod, K.; Emrick, T.; Gilardi, R. J. Am. Chem.
Soc. 1999, 121, 4111. (b) Eaton, P. E. Angew. Chem., Int. Ed. Engl. 1992,
1, 1421.
(5) (a) Tanielian, C.; Mechin, R. J. Phys. Chem. 1988, 92, 265. (b) Hoser,
S.; Capetanakis, F. P.; Kassner, C.; Stuhl, F. Chem. Phys. Lett. 1993, 208,
399. (c) Turro, N. J.; Chow, M.-F.; Kanfer, S; Jacobs, M. Tetrahedron Lett.
1981, 22, 3.
(
3
10.1021/ol802113j CCC: $40.75
Published on Web 11/17/2008
 2008 American Chemical Society

6
different efficiencies. Furthermore, different arrangements
of the hydrocarbon skeleton have been found to cause
1
7
considerable differences in the O
2
-quencher interaction.
Cubanes possess an unusual carbon skeleton that presents
1
2
an interesting case study of the deactivation of O .
Scheme 1. Cubane Derivatives Used in Steady-State Analysis
1
The absolute quenching rate constant for O
2
quenched
2
. phosphorescence decay rate
Pseudo-first-order 1O
Figure 1
constant (kobs) vs the cubane concentration for the determination
of the absolute O quenching rate constant by cubane (2) in CCl .
2 4
The decay traces at 1272 nm were generated by laser excitation
(532 nm) of the sensitizer TPP (2 µM). Inset: phosphorescence
decay traces in the absence (red) and presence (blue) of cubane
by cubane (2) was determined by a time-resolved phospho-
8
1
rescence quenching method.
sensitization with 5,10,15,20-tetraphenyl-21H,23H-porphyrin
TPP) and pulsed laser excitation (Nd:YAG laser at 532 nm,
2
O was generated by photo-
1
(
1
pulse width 10 ns). Decay traces of the O
2
phosphorescence
(
0.023 M).
at 1272 nm were recorded at different cubane concentrations
(
(
Figure 1, inset). The bimolecular quenching rate constant
3
-1 -1
k
q
) (6.8 ( 0.4) × 10 M s ) was extracted from the
slope of the plot of the pseudo-first-order decay rate constant
concentrations in CCl
4
. The Stern-Volmer plots for the
1
1
of the O
Figure 1). For these experiments, CCl
solvent because of its low volatility and the relatively long
2
phosphorescence versus the cubane concentration
quenching of O by a variety of cubane derivatives are
2
(
4
was selected as the
shown in Figure 2. The data for all of the cubanes studied
present good linear behaviors. In order to convert the
Stern-Volmer constants to bimolecular quenching constants,
1
9
lifetime of O
2
, which makes it a convenient solvent for
1
the determination of low quenching rate constant values that
are expected in this case.
k
q
, the Stern-Volmer constants were divided by the O
2
1
4
lifetime in CCl under our experimental conditions. A O
2
1
The O
2
rate constants for a series of cubane derivatives
lifetime of 31 ms was obtained using cubane (2) and the
previously determined absolute quenching rate constant
(Figure 1).
(
3-7) were determined relative to the quenching constant
of the unsubstituted cubane (2) using a steady-state chemi-
luminescence quenching method.
5
c,10
The thermal decom-
Table 1 shows that the experimentally determined k values
q
position of 1,4-dimethylnaphthalene-1,4-endoperoxide (1)
for the cubane derivatives fall in a very narrow range between
1
3
4
-1 -1
produces O
the chemiluminscence emission (phosphorescence) of O
at 1272 nm produced by thermolysis of 1 was utilized to
2
cleanly and in good yield. The quenching of
6.3 × 10 and 1.1 × 10 M s . The k constants follow
q
1
2
the expected magnitude order based on the number of
heteroatom-hydrogen bonds (C-H and O-H) present in the
molecule: cyanocubane (4) contains the fewest heteroatom-
hydrogen bonds and is found to have the smallest rate
constant, while biphenyl cubane (7) contains the most and
exhibits the largest rate constant.
1
1
determine Stern-Volmer quenching constants.
The quenching of the phosphorescence of O
1
2
was studied
in the presence of the cubanes listed in Scheme 1 at different
(
6) (a) Hurst, J. H.; Schuster, G. B. J. Am. Chem. Soc. 1983, 105, 5756.
b) Rodgers, M. A. J. J. Am. Chem. Soc. 1983, 105, 6201.
7) (a) Gassman, P. G.; Olson, K. D.; Walter, L.; Yamaguchi, R. J. Am.
A more appropriate description than that based on the
number of heteroatom-hydrogen bonds should also consider
the contribution of different types of heteroatom-hydrogen
bonds to the energy exchange. In pioneering research by
(
(
Chem. Soc. 1981, 103, 4977. (b) H o¨ ser, S.; Capetanakis, F. P.; Kassner,
C.; Stuhl, F. Chem. Phys. Lett. 1993, 205, 399. (c) Tanielian, C.; Mechin,
R. J. Phys. Chem. 1988, 92, 265. (d) Abdel-Shafi, A. A.; Worrall, D. R.;
Wilkinson, F. J. Photochem. Photobiol. A 2001, 142, 133.
6
a
6b
Hurst and Schuster and Rodgers, it was proposed that
individual rate constants for different functional groups can
be added to predict the bimolecular rate constant. Using this
(
8) Martinez, C. G.; Jockusch, S.; Ruzzi, M.; Sartori, E.; Moscatelli,
A.; Turro, N. J.; Buchachenko, A. L. J. Phys. Chem. A 2005, 109, 10216.
(
(
9) Schweitzer, C.; Schmidt, R. Chem. ReV. 2003, 103, 1685.
10) Ben-Shabat, S.; Itagaki, Y.; Jockusch, S.; Sparrow, J. R.; Turro,
q
group contribution method, k constants were calculated for
N. J.; Nakanishi, K. Angew. Chem., Int. Ed. 2002, 41, 814
11) Stern, O.; Volmer, M. Phys. Z. 1919, 20, 183.
.
(
all of the cubanes in Table 1 (k
q
′). Although the calculated
5510
Org. Lett., Vol. 10, No. 24, 2008

Table 2. Bimolecular Quenching Rate Constant for the
1
Quenching of O
2
by Some Reference Compounds
kq, M- s-1
1
kq, M-1 s-1
(literature values)
a
b
quencher
(this work)
3
4
hexadecane
9.5 × 10
1.20 × 10
3
4
1
1
a
-decanol
-nonanol
8.3 × 10
1.06 × 10
3
3
7.4 × 10
9.45 × 10
b
CCl4 as solvent. Quencher as solvent.
the overall quenching constant. This value is one order of
magnitude higher than that calculated by Rodgers for a
-1 -1
normal alkane C-H bond (90 M s ). Although vibrational
frequencies of strained alkane C-H bonds such as cubane
1
Figure 2
.
Stern-Volmer plots for the quenching of O
2
chemilu-
1
minescence by cubane derivatives 2-7 in CCl
4
. O
2
was generated
-1
12
are slightly higher in energy (2977 cm ) than in unstrained
by thermal decomposition of 1,4-dimethyl-1,4-endoperoxide (2 mM)
-
1
13
alkane C-H bonds such as hexane (2855 cm ), strain
alone probably cannot account for such large differences in
quenching rate constants. The higher frequency of saturated
C-H bonds in strained hydrocarbons is expected to enhance
at 22 °C.
q
k , but we believe that the effect of strain alone cannot
account for the observed quenching constants of the cubanes.
Table 1. Experimental and Calculated Bimolecular Quenching
1
Rate Constants for O
2
in the Presence of Cubane Derivatives
2
-7
Another factor that can be responsible for the quenching
1
of O
2
is the formation of a charge-transfer complex. Charge-
kq, exp
1
3
-1
transfer quenching of O
2
by amines, aromatic hydrocarbons,
(
10 M
no. of
kq′
kq′′
-
1
X-H bonds (10 M s-1
3
-1
a
(10 M s-1
)
3
-1
b
quencher
s
)
)
metal complexes, and metallofullerenes has been reported
3
9
in the literature with k
q
constants that range from 10 to 10
s . Darmanyan, et al. have shown that if the
ionization potential (IP) of the quencher is low enough (<8.5
7
5
6
3
2
4
a
11.0
8.6
8.3
8.1
6.8
6.3
16
14
12
10
8
4.90
4.02
3.10
1.18
0.72
0.70
9.8
8.9
8.0
6.1
5.6
5.6
-
1
-1 14
M
1
4a
eV) charge-transfer quenching can occur. For example,
the bimolecular quenching rate constant for charge-transfer
(kq,ct) for toluene (IP ) 8.82 eV, E1/2 ) 1.98 V) is
while the measured value
of kq,ct for p-bromoanisole (IP ) 8.49 eV, E1/2 ) 1.70 V)
15
7
ox
2
-1 -1
Calculated using Rodgers’ group contribution method for energy
estimated to be 9 × 10 M
s
b
exchange. Calculated assuming that the bimolecular quenching rate
constant is a contribution of the formation of a charge-transfer complex
ox
15
-1 -1 14a
3
-1 -1
3
between singlet oxygen (kq,ct, assumed to be 4.9 × 10 M
s
as explained
is 4.9 × 10 M s . Thus, kq,ct should increase as IP
in the text) and the cubane and energy exchange (kq′) as expressed in the
following equation: kq″ ) kq,ct + kq′.
ox ox
and redox potential (E1/2 ) decrease. The IP and E1/2 of
cubane are similar to those of p-bromoanisole, 8.46 eV and
1
5
16
1
.73 V, respectively, which suggests that a large contri-
bution to the rate constants in Table 1 comes from the
values correctly predict the relative order of the rate
constants, their magnitudes are underestimated by up to a
factor of 10 compared to the experimental values.
1
formation of a charge-transfer complex between O
2
and
1
•-
•+
cubane ( O
2
· · ·cubane ). Since the quencher molecules
6
b
used by Rodgers to extrapolate the contribution values for
C-H bonds were, in general, alkanes and alcohols with large
IP values, the formation of charge-transfer complexes was
not accounted for in that model.
To verify our technique, and to confirm the validity of
1
the reported values, we determined the k
q
constants for O
2
with several alkanes and alcohols in CCl
4
using the steady-
state chemiluminescence quenching method with cubane (2)
To obtain a better estimate of the bimolecular quenching
as a standard. These values are presented in Table 2 and
1
2
6b
constants for the quenching of O by cubanes, we took into
show good agreement with values determined by Rodgers.
account the contribution from the charge transfer complex
One factor that can contribute to the mismatch between
the calculated and experimental rate constants is that different
geometries of the hydrocarbon skeleton are known to quench
singlet oxygen with different efficiencies. Cubane is known
to have a unique carbon skeleton that is highly strained and
which possesses some double-bond character. This distinctive
(
12) Della, E. W.; McCoy, H. K.; Patney, H. K.; Jones, G. L.; Miller,
F. A. J. Am. Chem. Soc. 1979, 101, 7441.
(13) Pinkley, L. W.; Sethna, P. P.; Williams, D. J. Phys. Chem. 1978,
8
2, 1532.
(14) For amines and aromatic hydrocarbons, see: (a) Darmanyan, A. P.;
Jenks, W. S.; Jardon, P. J. Phys. Chem. A 1998, 102, 7420. (b) Abdel-
Shafi, A. A.; Wilkinson, F. J. Phys. Chem. A 2000, 104, 5747. For metals:
q
geometry may contribute differently to the k constant than
(
c) Rajadurai, S.; Das, P. K. J. Photochem. 1987, 37, 33. (d) Corey, E. J.;
a normal alkane C-H bond. Dividing the experimentally
determined rate constant of cubane (2) by 8 (the number of
C-H bonds in cubane), we determined that each cubane
Khan, A. U.; Ha, D. Tetrahedron Lett. 1990, 31, 1389. For metallofullerenes,
see: (e) Yanagi, K.; Okubo, S.; Okazaki, T.; Kataura, H. Chem. Phys. Lett.
2
007, 435, 306.
(
15) Against a standard calomel electrode (SCE) in acetonitrile.
-
1
-1
C-H bond should contribute approximately 850 M
s
to
(16) Gassman, P. G.; Yamaguchi, R. J. Am. Chem. Soc. 1979, 101, 1308.
Org. Lett., Vol. 10, No. 24, 2008
5511

ox
1
formation. Assuming that the IP and E1/2 values of the
2
nonradiative O deactivation by energy transfer to C-H
cubane derivatives (3-7) are not significantly affected by
the substituents and that the subsituents themselves do not
vibration modes cannot account for the value of the rate
constants determined. The formation of a charge-transfer
ox
1
contribute significantly to differences in the IP and E1/2
,
complex between the cubane and O
2
is supported by the
3
-1 -1
an estimated kq,ct of 4.9 × 10 M
s
was employed due
to the similarities between the IP and E1/2 values of cubane
and p-bromoanisole. Using this value together with Rodgers’
ionization and redox potentials of cubane, and this model
yields k constants in good agreement with the experimental
values. Thus, the k values can be reasonably explained by
ox
q
q
6
b
values for other common functional and hydrocarbon
groups, k constants were calculated that are more in
agreement with the experimentally obtained ones (Table 1,
′′). While this is a crude estimation since the exact IP’s
and E1/2 ’s are not known for the cubane derivatives, the
remarkably good match between the experimental and
calculated values is consistent with the contribution of these
two different deactivation mechanisms to the total bimo-
lecular quenching rate constant. This calculation could be
a combination of two distinct mechanisms: energy transfer
to C-H vibrational modes and charge-transfer quenching.
q
k
q
Acknowledgment. We thank Philip E. Eaton of the
University of Chicago for generously providing the cubane
samples and for his guidance throughout and the NSF for
its generous support of this research through Grant Nos. CHE
04-15516 and CHE 07-17518.
ox
Supporting Information Available: Time-resolved phos-
phorescence-quenching method and steady-state chemilu-
minescence quenching method to determine rate constants,
synthetic procedures, and characterization. This material is
available free of charge via the Internet at http://pubs.acs.org.
modified to a higher degree of precision if the exact IP and
ox
E
1/2 were known for each derivative in order to determine
a more accurate contribution of kq,ct to the overall quenching.
1
In conclusion, the quenching of O
2
by different cubane
constants
are in the order of 10 -10 M s-1. A classical model of
derivatives has been studied. The determined k
q
3
4
-1
OL802113J
5512
Org. Lett., Vol. 10, No. 24, 2008