Phenylethanol derivatives as triplet sensitizers for 1-azidoadamantane

Article abstract of DOI:10.1002/poc.1922

Laser flash photolysis, phosphorescence and density functional calculations were used to characterize the triplet excited states of phenylethanol derivatives 1, 2, 3 and 4. In acetonitrile, the triplet excited states of these alcohols were formed with rat

Full text of DOI:10.1002/poc.1922

Research Article
Received: 10 March 2011,
Revised: 10 July 2011,
Accepted: 11 July 2011,
Published online in Wiley Online Library: 22 August 2011
(wileyonlinelibrary.com) DOI 10.1002/poc.1922
Phenylethanol derivatives as triplet sensitizers
for 1-azidoadamantane
Ranaweera R. A. Upul Ranaweeraa, Sridhar Rajama and
Anna D. Gudmundsdottira*
Laser flash photolysis, phosphorescence and density functional calculations were used to characterize the triplet ex-
cited states of phenylethanol derivatives 1, 2, 3 and 4. In acetonitrile, the triplet excited states of these alcohols were
formed with rate constants on the order of ~107 s–1 and decayed with rate constants between 105 and 106 s–1. The
energies of these triplet excited states were between 80 and 83 kcal/mol. Furthermore, 3 can be used as a triplet
sensitizer for alkyl azides to form triplet nitrene intermediates. Copyright © 2011 John Wiley & Sons, Ltd.
Keywords: alky nitrene; laser flash photolysis; triplet sensitization
Synthesis of 1-(4-methoxy-2,5-dimethylphenyl) ethanone
INTRODUCTION
To
a solution of 2-methoxy-1,4-dimethylbenzene (4.80 g,
Triplet sensitization makes it possible to form various triplet ex-
cited states and reactive intermediates that cannot be formed by
direct irradiation. For example, direct irradiation of alkyl azides
does not lead to the formation of triplet alkyl nitrenes but rather
to imine products either via a concerted reaction of the singlet ex-
cited state of the alkyl azide or by the formation of a singlet alkyl
nitrene intermediate, which rearranges as shown in Scheme 1.[1–4]
Thus, intersystem crossing, either from the singlet excited state
of the alkyl azide to its triplet excited state or from the singlet
nitrene to its triplet configuration, must be slower than the for-
mation of the imine product. However, triplet alkyl nitrenes can
be formed by intermolecular and intramolecular sensitization
of alkyl azides with acetophenone derivatives.[5–10] Acetophe-
none derivatives are excellent triplet sensitizers for alkyl azides
because they absorb light at longer wavelengths than alkyl
azides. In addition, acetophenone derivatives undergo intersys-
tem crossing to their triplet states efficiently, with rate constants
on the order of 1011 s–1 and with quantum yields approaching
unity.[11–13] In our continuing effort to study triplet intermediates
using sensitization, we characterized the triplet states of pheny-
lethanol derivatives and showed that they can be used as triplet
sensitizers for alkyl azides to form triplet nitrenes.
Here, we describe the characterization of the first excited triplet
state (T1) of phenylethanol derivatives 1, 2, 3 and 4 by phospho-
rescence, laser flash photolysis and density functional calcula-
tions (Scheme 2). The T1 of 1, 2, 3 and 4 are located between 83
and 80kcal/mol above their ground state (S0). The T1 of these phe-
nylethanol derivatives have lifetimes of a few microseconds in
acetonitrile, but the intersystem crossing rate constants to form
the triplet excited states are only on the order of ~107 s–1. Alco-
hol 3 can be used as an efficient sensitizer for azidoadamantane
to form a triplet alkyl nitrene, whereas 1, 2 and 4 are less suitable.
0.035 mol) in dichloromethane (10 mL) at 0 ꢀC was added
CH3COCl (2.75 g, 0.035 mol) in dichloromethane (10 mL). AlCl3
(9.34 g, 0.070 mol) was added in small portions to the solution.
The resulting mixture was stirred for 4 h and poured into an ice
bath. The mixture was acidified by adding a saturated solution
of NH4Cl. The reaction mixture was extracted with dichloro-
methane and washed with water. The organic layer was dried
with anhydrous MgSO4 and the solvent was removed under vac-
uum to yield 1-(4-methoxy-2,5-dimethylphenyl)ethanone (2.68 g,
0.015 mol, 43% yield).
Synthesis of 1-(4-methoxy-2,5-dimethylphenyl)ethanol (4)
CeCl3.7H2O (7.45 g, 0.02 mol) was added to a solution of 1-(4-
methoxy-2,5-dimethylphenyl)ethanone (2.68 g, 0.015 mol) in
methanol at ambient temperature while stirring.[14] The reaction
mixture was cooled to 0 ꢀC and NaBH4 (0.76 g , 0.020 mol) was
added in small portions. The resulting mixture was stirred for
25 min and a saturated solution of NH4Cl (15 mL) added. The re-
action mixture was extracted with diethyl ether and washed with
water. The organic layer was dried with anhydrous MgSO4 and
the solvent was removed under vacuum to give 4 (2.08 g). The
crude was purified on silica column eluted with ethyl acetate in
hexane mixture (1:4) to yield 4 (0.74 g, 4.1 mmol, 27% yield).
1H NMR (CDCl3, 400 MHz): d 1.45 (d, J = 5 Hz, 3H), 1.62 (br s,
1H), 2.20 (s, 3H), 2.33 (s, 3H), 3.81 (s, 3H), 5.07 (q, J = 5 Hz, 1H),
6.59 (s, 1H), 7.26 (s, 1H) ppm. GC/MS (EI): m/z 180 (M+), 162
(100%), 147, 137, 131, 122, 115, 105, 91, 77, 65, 51.
* Correspondence to: Anna Gudmundsdottir, Department of Chemistry, Univer-
sity of Cincinnati, Cincinnati, OH, USA.
E-mail: annag@uc.edu
SYNTHESIS OF STARTING MATERIALS
a R. R. A. Upul Ranaweera, S. Rajam, A. D. Gudmundsdottir
Department of Chemistry, University of Cincinnati, Cincinnati, OH, 45221-0172,
USA
1-Azidoadmantane and phenylethanol derivatives 1, 2 and 3 are
commercially available. We prepared 4 as described below.
J. Phys. Org. Chem. 2011, 24 902–908
Copyright © 2011 John Wiley & Sons, Ltd.

PHENYLETHANOL DERIVATIVES AS TRIPLET SENSITIZERS
*
oxygen for 15 min. The rate constants were obtained by fitting
an average of three to eight kinetic traces.
h
ν
R
N
R
N
R
NH
+ N2
Quenching studies of T1 of 3 with azidoadamantane were
done as follows. A stock solution of 25 mM 3 (0.760 g, 5 mmol)
in acetonitrile (200 mL) was prepared. A series of solutions were
made by dissolving 4.6 mg, 8.9 mg, 24.0 mg, 31.6 mg, and
43.8 mg of azidoadamantane in 25 mL of the stock solution of
3 to: yield 1.0 mM, 2.0 mM, 5.0 mM, 7.0 mM and 10.0 mM solu-
tions of azidoadamantane.
Singlet
Excited State
Triplet
Sensitizer
R
N:
R
N:
Singlet
Nitrene
Triplet
Nitrene
PHOSPHORESCENCE
The phosphorescence spectra were obtained on a phosphori-
meter in ethanol glasses at 77 K. The solutions were irradiated
at 260 nm and the emission spectra recorded between 300 and
600 nm.
R
N
N
R
Scheme 1. Photoreactivity of Alkyl Azides
TRIPLET SENSITIZATION
COMPUTATIONAL DETAILS
A mixture of 1, 2, 3 or 4 (0.2 mmol) and 1-azidoadamantane
(35 mg, 0.2 mmol) was dissolved in CH3CN (2 mL, 0.1 M solution),
and the resulting solution was purged with argon for 5 min.
This solution was irradiated through a Pyrex (Corning, Lowell,
MA, USA) filter, and the reaction mixture was analyzed by gas
chromatography-mass spectroscopy (GC-MS).
The calculations were performed using density functional theory
(DFT) at the Becke, three-parameter, Lee–Yang–Parr (B3LYP) level
of theory and with the 6-31 + G(d) basis sets as implemented in
GAUSSIAN 09 (Gaussian Inc., Wallingford, CT, USA).[15–17] It has been
shown that the B3LYP method can successfully predict energies
for open-shell systems such as triplet nitrenes.[18] All of the ge-
ometries were optimized at the B3LYP/6-31 + G(d) level of the-
ory, and for each stationary point, the second derivative of the
energy was calculated to confirm that these structures represent
local energy minima. Solvation effects were evaluated for aceto-
nitrile and methanol using the integral equation formalism polar-
izable continuum model with complete geometry optimizations
and vibrational frequency analyses at the B3LYP/6-31 + G(d) level
of theory.[19–22] The energies of the excited states and the ab-
sorption spectra were calculated using time-dependent density
functional theory (TD-DFT).[23–27] Spin contamination was negli-
gible for all of these open-shell molecules. All of the calculations
were performed at the Ohio Supercomputer Center.
RESULTS AND DISCUSSION
Product studies
Prolonged irradiation of 1, 2, and 3 through a Pyrex filter in both
argon-saturated and oxygen-saturated solution did not yield any
photoproducts. In comparison, photolysis of 4 did result in the
formation of a small amount of dimeric products that we did
not characterize further.
We photolyzed a 1:1 solution of 0.1 M 1, 2, 3 or 4 and azidoa-
damantane through a Pyrex filter. We followed the reaction with
GC-MS and kept the conversion below 20%. Sensitization of azi-
doadamantane with 3 yielded azo-dimer 5 as its only product,
whereas sensitization with 1 and 4 resulted in the formation of
5 and 6 (Scheme 3). Dimer 5 has been shown to form by the di-
merization of two triplet alkyl nitrene intermediates,[7] whereas 6
is formed via the singlet reactivity of azidoadamantane to form 7,
which is trapped with an alcohol.[29,30] Photolysis of azidoada-
mantane with 2 or toluene as a sensitizer did not yield any
photoproducts. The product studies show that 3 is an efficient
triplet sensitizer for azidoadamantane, because sensitization
with 3 completely bypassed the singlet reactivity of azidoada-
mantane. In comparison, 1 and 4 were not as efficient triplet sen-
sitizers as they gave products that can be attributed to both
LASER FLASH PHOTOLYSIS
Laser flash photolysis was performed with an Excimer (Lambda
Physik, Inc.Fort Lauderdale, FL, USA) laser (308 nm, 17 ns), using
a previously described system.[28] Stock solutions of alcohols 1,
2, 3 and 4 in methanol and acetonitrile were prepared with spec-
troscopic grade solvents such that the solutions had absorptions
between 0.3 and 0.8 at 308 nm. Typically, ꢁ2 mL of the stock so-
lution was placed in a 48-mm-long quartz cuvette cell (10 mm
10 mm in cross-section) and purged with argon for 5 min or
OH
OH
OH
OH
O
O
1
2
3
4
Scheme 2. Molecular Structure of Alcohols 1, 2, 3 and 4
J. Phys. Org. Chem. 2011, 24 902–908
Copyright © 2011 John Wiley & Sons, Ltd.
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R. R. A. UPUL RANAWEERA, S. RAJAM AND A. D. GUDMUNDSDOTTIR
Sensitizer
N:
N
N3
N
Triplet Nitrene
5
hν
Direct
R-OH
NH
N
R
O
R = 1 or 4
7
6
Scheme 3. Triplet Sensitization of Azidoadamantane
singlet and triplet reactivity of azidoadamantane, whereas sensi-
tization with 2 and toluene was not successful.
state of toluene. We optimized the ground state (S0) of 1, 2,
3, 4 and toluene and performed TD-DFT calculations to estimate
the energies of their first excited singlet and triplet states (S1 and
T1). We found that the S1 are located 122 kcal/mol above the S0
for 1 and toluene. The S1 of 2 is located 116 kcal/mol above its
S0 whereas the S1 of 3 and 4 are 114 kcal/mol above their S0
(Table 2). The T1 of 1, 2, 3, 4 and toluene were placed between
82 and 86 kcal/mol above their S0. Inspection of the molecular
orbitals demonstrates that S1 is due to a (p,p*) electronic transi-
tion. In comparison, the T1 in 1, 2 and 3 are due to a mixed elec-
tronic transition from a p-orbital and the lone pair on the alcohol
UV absorption
To better evaluate the ability of 1, 2, 3 and 4 as triplet sensitizers
for azidoadamantane; we compared the absorption spectra of
alcohols 1, 2, 3 and 4 with the absorption spectra of toluene
and acetophenone (Table 1). Alcohols 3 and 4 have an absorp-
tion band at ~280 nm that trails out to 300 nm (Figure 1),
whereas 1 and 2 have weaker absorption bands at a somewhat
shorter wavelength, similar to the absorption spectrum of tolu-
ene. The absorption of 1, 2, 3 and 4 above 300 nm is weaker than
for the acetophenone, which has an (n, p*) absorption band
around 320 nm (e = 50, Table 1) [31], and has been shown to be
an efficient triplet sensitizer for azidoadamantane[7]. However,
we conclude that toluene, 1 and 2 are not as effective triplet sen-
sitizers because their ground state absorption is similar to azi-
doadamantane, which has a weak absorption band at 286 nm
(e = 25 M–1 cm–1).[32]
1.0
1
2
3
4
0.5
0.0
Calculations
240
260
280
300
320
340
360
380
400
To further analyze the ability of 1, 2, 3 and 4 to be used as
sensitizers for azido compounds, we calculated their triplet
excited states and compared them with the triplet excited
Wavelength [nm]
Figure 1. UV absorption spectra for 0.20 mM solutions of 1, 2, 3 and 4 in
ethanol
Table 1. The ground state UV absorption lmax and e for 1,[33]
2, 3 and 4
Table 2. Calculated energies in kcal/mol for S1 and T1 of 1, 2,
λmax (e)
λmax (e)
λmax (e)
3, 4 and toluene
nm
nm
nm
TD-DFT
Optimized T1
(M–1 cm–1) (M–1 cm–1)
(M–1 cm–1)
S1
T1
T2
Acetophenone[34]
320 (50)
278 (1110)
240
(1.3 Â 104)
GP GP EtOH MeCN GP GP EtOH MeCN
Toluene[34]
1
2
3
262 (174)
259 (300)
277 (250)
282 (2200) 276 (2400)
284 (2600) 278 (2600)
286 (25)
208 (2460)
252 (300)
268 (300)
1
2
3
4
122 86 86
116 83 84
114 84 85
114 82 82
122 86 86
87
84
85
82
86
106 86 87
102 80 80
94 80 80
96 78 78
105 85 85
87
80
80
78
85
4
1-
Toluene
Azidoadamantane.[32]
GP: Gas phase.
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Copyright © 2011 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 902–908

PHENYLETHANOL DERIVATIVES AS TRIPLET SENSITIZERS
moiety into a p*-orbital (Figure 2), whereas the T1 of 4 is mainly
due to a (p,p*) electronic transition, similar as observed for T1 of
toluene. Solvation in ethanol or acetonitrile did not change the
calculated energies of S1 and T1 of 1, 2, 3, 4 and toluene
significantly.
In addition, we optimized the T1 of 1, 2, 3, 4 and toluene
(Figure 3). The most significant difference between the opti-
mized S0 and the T1 of 1, 2, 3 and 4 is that the carbon–carbon
bonds in the aromatic ring are no longer equivalent; some of
the bonds became longer and others shorter, as has been observed
for the T1 of benzene and toluene.[35] The optimized structure of
the T1 of toluene is similar to what Cogan et al. have reported
using Complete Active Space Self Consistent Field (CASSCF)
calculations.[36] We found that the energies obtained from the
optimized structures were 3–4 kcal/mol lower in energy than
those obtained by TD-DFT calculations for 2, 3 and 4, whereas
the energy calculated for the T1 of 1 and toluene was similar
by both methods. Thus, unrestricted Becke, three-parameter,
-orbital iv
-orbital iii
n-orbital i
-orbital ii
i
ii
iii iv
Figure 2. The orbitals involved in the lowest energy electronic transitions for the T1 of 3 (TD-DFT in CH3CN)
1.36
1.41
1.38
1.53
1.43
1.50
1.44
1.45
1.50
1.51
1.50
1.49
1.36
1.36
1
2
1.40
1.51
1.39
1.40
1.39
1.50
1.50
1.38
1.50
1.40
1.49
1.51
1.39
1.38
1.41
1.53
1.53
1.39
1.38
1.51
1.41
3
4
Toluene
C-C bond distances are in Å
Figure 3. Optimized structures of the T1 of 1, 2, 3, 4 and toluene
J. Phys. Org. Chem. 2011, 24 902–908
Copyright © 2011 John Wiley & Sons, Ltd.
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R. R. A. UPUL RANAWEERA, S. RAJAM AND A. D. GUDMUNDSDOTTIR
Lee–Yang–Parr (UB3LYP) optimization of 1, 2, 3 and 4 yielded ener-
gies that are in good agreement with those obtained from TD-DFT
calculations. We have previously shown that UB3LYP significantly
underestimates the adiabatic energy of triplet ketones with (n,p*)
configuration, whereas UB3LYP assessment of the adiabatic en-
ergy of triplet ketones with (p,p*) configuration is excellent.[37,38]
The DFT calculations show that 1, 2, 3 and 4 have their T1 with
similar or slightly lower energy than T1 of toluene.
Table 3. The (0,0) bands obtained from phosphorescence
spectra of 1, 2, 3 and 4 in ethanol at 77 K
(0,0) nm
kcal/mol
Acetophenone[39]
74
83
83
80
81
80
Toluene[39]
1
2
3
4
344
357
355
357
Phosphorescence
We measured the phosphorescence of 1, 2, 3 and 4 in ethanol
glasses at 77 K. These spectra are shown in Fig. 4, and the ener-
gies of the (0,0) bands are listed in Table 3. The emission spectra
for 1 and 2 have a resolved fine structure that makes it possible
to locate their (0,0) transitions, whereas the emission for 3 and 4
are not sufficiently resolved to locate the (0,0) transitions. There-
fore, we used the onset of the emissions at the shortest wave-
length as an estimate for the (0,0) transitions for 3 and 4. The
intensity of the phosphorescence was somewhat lower for 4
than 1, 2 and 3.
The measured energies of the T1 in 1, 2, 3 and 4 in ethanol
glasses and the calculated values are in good agreement. The
calculated energies of the T1 for 1, 2, 3 and 4 are within
~3 kcal/mol of the measured energies. The energies of the T1 in
1, 2, 3 and 4 are similar to the reported energies of the T1 in tol-
uene (83 kcal/mol) and benzene (81 kcal/mol).[39] Furthermore,
the calculated energy of T1 in toluene fits well with the measured
one. Thus, the phosphorescence studies verify that DFT calcula-
tions are suitable for optimizing the triplet excited state of phe-
nyl ethanol derivatives.
LASER FLASH PHOTOLYSIS
We performed laser flash photolysis of 1, 2, 3 and 4 to further
characterize their T1. Laser flash photolysis of 1 in acetonitrile
resulted in a transient spectrum with lmax at 320 and 350 nm
(Figure 5). The transient absorption was quenched in an oxy-
gen-saturated solution. In addition, the rate constant for the
transient formation at 320 nm was 2 Â 107 s–1, whereas the tran-
sient decayed with a rate constant of 3 Â 105 s–1. Based on the
TD-DFT calculations, we assigned this transient to the T1 of 1. La-
ser flash photolysis of 1 in methanol resulted in a similar tran-
sient spectrum, whereas the decay rate constant for the T1 of 1
was somewhat slower at 4 Â 104 s–1.
Laser flash photolysis of 2 in argon-saturated acetonitrile
resulted in a transient spectrum with lmax around 360 nm, which
we assigned to the T1 of 2 based on the TD-DFT calculations
(Figure 6). We did not measure the transient absorption below
330 nm because of interference by the fluoroscence from S1 of
2. This transient absorption was quenched in oxygen-saturated
acetonitrile. The transient was formed with a rate constant of
1.7 Â 107 s–1 and decayed at a rate constant of 1.3 Â 106 s–1.
Phosphorescence
2x10
0.04
0.02
0.00
1
(0,0) = 344 nm
0
300
300
350
350
400
450
500
550
600
Wavelength [nm]
0
50
100
150
200
250x10
1.0x10
0.5
0.0
Time [s]
(0,0) = 357 nm
0.10
0.05
0.00
400
450
500
550
600
Wavelength [nm]
1.0x10
300
350
400
450
500
550
600
of 1
(0,0) = 355 nm
0.5
0.0
Wavelength [nm]
0.02
0.01
0.00
300
300
350
350
400
450
500
550
600
T
1
Wavelength [nm]
1.0x10
0.5
0.0
300
350
400
450
500
550
600
(0,0) = 357 nm
Wavelength [nm]
400
450
500
550
600
Figure 5. Laser flash photolysis of 1. (a) Kinetic trace obtained at 320 nm
in argon-saturated methanol; (b) transient spectrum in argon-saturated
acetonitrile immediately after the laser pulse; and (c) calculated TD-DFT
spectrum for the T1 of 1 in acetonitrile
Wavelength [nm]
Figure 4. Phosphorescence spectra for (a) 1, (b) 2, (c) 3 and (d) 4
obtained in ethanol at 77 K with 260 nm irradiation
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PHENYLETHANOL DERIVATIVES AS TRIPLET SENSITIZERS
0.10
0.05
0.00
0.5
0.0
2
4
6
8
10x10
5
10
15
20
25x10
Time [s]
Time [s]
0.1
0.0
0.5
0.0
-0.1
350
400
450
500
550
600
Wavelength [nm]
350
400
450
500
550
600
Wavelength [nm]
0.10
0.05
0.06
0.04
0.02
0.00
T
of 2
1
T
of 4
1
350
400
450
500
550
600
Wavelength [nm]
350
400
450
500
550
600
Wavelength [nm]
Figure 6. Laser flash photolysis of 2 in argon-saturated acetonitrile. (a)
Kinetic trace obtained at 370 nm; (b) transient spectrum obtained right
after the laser pulse; and (c) calculated TD-DFT spectrum for the T1 of 2
in acetonitrile
Figure 8. Laser flash photolysis of 4 in argon-saturated acetonitrile. (a)
Kinetic trace obtained at 460 nm; (b) transient spectrum obtained imme-
diately after the laser pulse; and (c) calculated TD-DFT spectrum for the T1
of 4 in acetonitrile
Laser flash photolysis of 3 resulted in a broad transient spec-
trum with lmax at 410 nm (Figure 7). This transient was formed
faster than the resolution of the laser flash apparatus (17 ns),
but it was quenched in oxygen saturated-acetonitrile. The rate
constant for the decay of this transient was measured at
410 nm and found to be 4.6 Â 105 s–1.
Thus, laser flash photolysis allowed for the direct detection of
the T1 of 1, 2, 3 and 4 at ambient temperature. The T1 of 1, 3 and
4 have lifetimes of several microseconds in acetonitrile, whereas
the T1 of 2 is somewhat shorter lived. We were able to measure
the rate constants for forming T1 of 1, 2 and 4 directly, and they
are on the order of ~107 s–1. This is similar to the reported rate
constants for the formation of the T1 of benzene and toluene.
Furthermore, laser flash photolysis of p-methylphenylethanol
has been reported to produce its T1, which has an absorption be-
tween 300 and 400 nm and a lifetime of 2 ms.[40]
Laser flash photolysis of 4 resulted in broad transient absorp-
tion as shown in Fig. 8, and we assigned it to the T1 of 4. This
transient absorption was quenched efficiently in oxygen-satu-
rated acetonitrile. The transient was formed with a rate constant
of 7 Â 106 s–1 and it decayed with a rate constant of 2 Â 105 s-1.
0.10
0.05
0.00
A
0.2
0 mM
1 mM
2 mM
5 mM
7 mM
10 mM
0.1
0.0
5
10
15
20
25x10
Time [s]
0.10
0.05
0.00
0
5
10
15x10
Time [s]
B
350
350
400
450
500
550
600
3x10
Wavelength [nm]
0.04
0.02
0.00
2
1
0
T
of 3
1
400
450
500
550
600
Wavelength [nm]
0.000
0.005
0.010
[Azidoadamantane] M
Figure 7. Laser flash photolysis of 3 in argon-saturated acetonitrile. (a)
Kinetic trace obtained at 410 nm; (b) transient spectrum immediately
after the laser pulse; and (c) calculated TD-DFT spectrum for the T1 of 3
in acetonitrile
Figure 9. (a) Kinetic traces at 400 nm for the T1 of 3 at various azidoada-
mantane concentrations. (b) The decay rate constants for the T1 of 3 at
various azidoadamantane concentrations
J. Phys. Org. Chem. 2011, 24 902–908
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R. R. A. UPUL RANAWEERA, S. RAJAM AND A. D. GUDMUNDSDOTTIR
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Because 3 and 4 have similar S0 absorption, but 4 is not as an
efficient sensitizer as 3, we propose that the smaller rate con-
stant for intersystem crossing in 4 causes it to be both a singlet
and triplet sensitizer for azidoadamantane. Furthermore, the
shorter lifetime of the T1 of 2 causes it to be a less effective sen-
sitizer than 1.
We measured the rate for quenching the T1 of 3 with azidoa-
damantane (Figure 9). The decay rate constant of T1 of 3, in-
creased with increasing concentration of azidoadamantane. A
plot of the decay rate constants for T1 of 3 versus the azidoada-
mantane concentration was fitted with a straight line, with a
slope of 1.6 Â 108 M–1 s–1. The rate for azidoadamantane quench-
ing the T1 of 3, is similar as to what is been reported as the rate of
azidoadamantane quenching the T1 of acetophenone.[7]
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CONCLUSION
We used phosphorescence and laser flash photolysis to charac-
terize the T1 of 1, 2, 3 and 4. The T1 are formed with rate con-
stants on the order of ~107 s–1 and have lifetimes of several
microseconds in solution. The energies of the T1 are between
80 and 83 kcal/mol, which makes them valuable as high-energy
triplet sensitizers. However, only 3 is an efficient triplet sensitizer
for azidoadamantane, because its intersystem crossing rate con-
stant is fast enough to prevent singlet sensitization from com-
peting with the triplet sensitization. In comparison the slow
intersystem crossing in 4, results in both singlet and triplet sen-
sitization of azidoadamantane. Alcohols 1 and 2 are not as effi-
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does not differ sufficiently from azidoadamantane in addition
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We thank the National Science Foundation (CHE-0703920) and
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U. R. is grateful to the University of Cincinnati Research Council
for its support and the UC Chemistry Department for a Harry
Mark Fellowship.
We wish Professor Robert Moss a happy 70th Birthday. We
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to the field of reactive intermediates over the years. We wish him
continuing success in exploring both reactive intermediates and
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