Syntheses and properties of the new electron transfer sensitizers 4,2/-pyrylogens

Article abstract of DOI:10.1021/ol802763r

(Chemical Equation Presented) The syntheses and characterizations of the 4,2′-regioisomers of the dicationlc pyrylogen electron transfer sensitizers are reported. The electrochemical and photophysical properties of these sensitizers are compared to the pr

Full text of DOI:10.1021/ol802763r

ORGANIC
LETTERS
2009
Vol. 11, No. 3
685-688
Syntheses and Properties of the New
Electron Transfer Sensitizers
4,2′-Pyrylogens
Edward L. Clennan* and Ajaya Kumar Sankara Warrier
Department of Chemistry, UniVersity of Wyoming, 1000 East UniVersity AVenue,
Laramie Wyoming 82071
clennane@uwyo.edu
Received November 29, 2008
ABSTRACT
The syntheses and characterizations of the 4,2′-regioisomers of the dicationic pyrylogen electron transfer sensitizers are reported. The
electrochemical and photophysical properties of these sensitizers are compared to the previously reported 4,4′-pyrylogens.
.
diffusive separation thereby enhancing inhibition of RET.
This new approach is an extension of the charge shift
reaction5 which previously was restricted to singly charged
sensitizers that generate nonattractive radical/radical-cation
pairs. Although this approach was successful, RETs in these
nonattractive radical/radical-cation pairs were far from
completely eliminated and separation quantum yields sig-
nificantly less than one were still observed.6 Consequently,
efforts to extend this concept to dicationic sensitizers to
generate mutally replusive ions are warranted. Therefore, we
report here the syntheses and characterizations of the
previously unknown 4,2′-isomers, 2a-c2+, of the 4,4′-
pyrylogens4 and their photophysical and electrochemical
properties.
Photoinduced electron transfer (PET) reactions have become
a standard tool in the synthetic chemist’s bag of utensils.1
The unique reactivities exhibited by open shell species
generated in these reactions have provided access to synthetic
transformations that are difficult or impossible to achieve
by any other method.2 Commercialization of PET reactions,
however, are impeded by the cost associated with their low
quantum yields. In many cases, the low effiencies can be
attributed to energy wasting return electron transfer (RET)
that competes with the desired reaction.3
To suppress detrimental RET, we recently introduced 4,4′-
pyrylogens, 12+, as a new class of sensitizer.4 These
sensitizers were designed to generate mutually replusive
radical-cations that would exhibit an increased rate of
These new pyrylogens were synthesized as shown in
Scheme 1.7 In the first step, condensation of two equivalents
of a phenone, presumably via an R,ꢀ-unsaturated intermedi-
(1) Kravarnos, G. J. Fundamentals of Photoinduced Electron Transfer;
VCH Publishers Inc.: Cambridge, UK, 1993.
(2) Ohkubo, K.; Iwata, R.; Miyazaki, S.; Kojima, T.; Fukuzumi, S. Org.
Lett. 2006, 8, 6079–6082.
(3) Gould, I. R.; Moser, J. E.; Armitage, B.; Farid, S. Res. Chem.
Intermed. 1995, 21, 793–806.
(5) Gould, I. R.; Moser, J. E.; Armitage, B.; Farid, S.; Goodman, J. L.;
Herman, M. S. J. Am. Chem. Soc. 1989, 111, 1917–1919.
(6) Todd, W. P.; Dinnocenzo, J. P.; Farid, S.; Goodman, J. L.; Gould,
I. R. J. Am. Chem. Soc. 1991, 113, 3601–3602.
(4) Clennan, E. L.; Liao, C.; Ayokosok, E. J. Am. Chem. Soc. 2008,
130, 7552–7553.
10.1021/ol802763r CCC: $40.75
Published on Web 12/22/2008
2009 American Chemical Society

ate, generates a 1,5-diketone that is readily converted to the
pyridinium ion with Meerwein’s salt. Oxidative cyclization
of the 1,5-diketo-pyridinium salt was best accomplished using
in situ generated trityl cation in a mixture of acetic anhydride
and 48% HBF4.
Scheme 1. Syntheses of 4,2′-Pyrylogens
Figure 1
. (a) Face-on and (b) edge-on perspectives of the X-Ray
structure of 2a2+
.
logen 12+ regioisomers, the radical cations and the neutral
redox partners are stable on the cyclic voltammetry (CV)
time scale even at scan rates of 50 mV/sec. However, there
are also very distinctive differences in the CV behavior of
the two regioisomers. For example, the first reduction
potential of 2a2+ to form the radical cation is 110 mV
cathodic of the first reduction of 12+. The reduction potentials
(Table 1) to form the radical cations (E1/2(1)) are also more
sensitive to substituent effects in the 4,2′- (F ) 0.92; r2 )
0.9978) than in the 4,4′- (F ) 0.34; r2 ) 0.9988) regioiso-
mers.4 In addition, the 4,2′ pyrylogen radical cations and
neutral redox partners react rapidly with oxygen on the CV
time scale even at 500 mV/sec while the 4,4′-regioisomer,
12+, is unreactive even at scan rates as slow as 50 mV/sec.
The spectral data, especially the appearance of a diagnostic
two proton singlet for the pyrylium ring hydrogens at 8.68
ppm in 2a2+, 8.30 ppm in 2b2+, and 8.90 ppm in 2c2+, and
the disappearances of the carbonyl peak at 198.1 ppm (2a2+),
197.4 (2b2+), and 198.4 (2c2+) in the 13C NMR, and the CO
stretch in the IR at 1684 cm-1 (2a2+), 1672 cm-1 (2b2+),
and 1700 cm-1 (2c2+) in the 1,5-dione precursor provides
strong evidence for formation of the pyrylogen. (See Sup-
porting Information for complete spectral data.) This was
confirmed by an X-Ray structure of 2a2+·2BF4- (Figure 1).
A striking feature of the structure is the significant twist of
the pyridinium ring (65.8°) out of the plane of the pyrylium
cation in comparison to the 2- and 6-phenyl rings (2.5° and
10.7°). We attribute this to the steric interaction between
the N-methyl group and the pyrylium ring rather than to
crystal packing forces. For comparison, the pyridinium ring
in 12+ is only twisted out of the pyrylium ring plane by
41.5°.4 This contention is also supported by calculations
using the B3LYP/6-31G(d) model which predicts twist
angles of 53.2°, 78.4°, 73.7°, 75.3°, 74.7°, and 78.6° for 12+,
2a2+, the (ii), (io), and (oo) rotomers of 2b2+, and 2c2+,
respectively.8 (See Supporting Information for more detailed
computed structural data.)
Scheme 2. Redox States of the 4,2′-Pyrylogens
To understand these differences, we have computationally
examined all three redox states using the B3LYP/6-31G(d)
model. With this model, 2a2+ is 8.11 kcal/mol less stable
than its isomer 12+ but 2a+• is 9.37 kcal/mol less stable than
1+•. These calculated energy differences require that the
dication/radical-cation energy gap be 1.26 kcal/mol smaller
for 12+ than for 22+ nearly identical to the 110 mV (2.53
kcal/mol) difference in reduction potentials. We suggest that
the higher energies of 2a2+ and 2a+• than their 4,4′-
counterparts reflect the loss of delocalization energy in the
more highly twisted 4,2′-isomers (i.e., the inter-ring dihedral
angles in 2a2+ (78.45°) and 2a+• (35.41°) are significantly
larger than in 12+ (41.48°) and 1+•(3.91°)).
Electrochemical Behavior. The 4,2′-pyrylogens in anal-
ogy to the closely related viologens exist in three readily
accessible redox states as shown in Scheme 2. In both the
4,2′-pyrylogens, 22+, and N-methyl-2,6-diphenyl-4,4′-pyry-
(7) Katritzky, A. R.; Adamson, J.; Elisseou, E. M.; Musumarra, G.; Patel,
R. C.; Sakizadeh, K.; Yeung, W. K. J. Chem. Soc., Perkin Trans. II 1982,
1041–1048.
686
Org. Lett., Vol. 11, No. 3, 2009

The differences in sensitivity to substituent effects we
attribute to a greater decrease in the positive charge upon
12+ and 2a2+ under the same reducing conditions also
generated single line ESR spectra with g-values of 2.0215
and 2.0038, respectively.10
Table 1. Photophysical and Electrochemical Data for 1, 2a-c,
and 3
g
3+
12+
2a2+
2b2+
2c2+
E1/2 (1)a 0.17
0.06
-0.08
-0.45
623
628
63.6 ( 8
113
52
49
0.22
-0.34
481
550
19.6 ( 4
71
64
56
-0.35
-1.53
465
E1/2 (2)a -0.35
-0.41
511
b
λF
λP
533
565
c
567
520
d
τP
35.6 ( 0.2 39.5 ( 2
225 ( 3
64
∆λe
93
76
61
54
E (S1) 59
E (T1) 54
65f
53f
ΦF
0.18 ( 0.01 0.54 ( 0.01 0.048 ( 0.01 0.44 ( 0.04 0.60f
a In volts vs SCE. b In nm at 298 K. c In nm in EtOH/HCl(g) at 77 K.
d In milliseconds (ms) at 77 K in EtOH/HCl(g). e Stokes shift in nm.
f Miranda, M. A.; Garcı´a, H. Chem. ReV. 1994, 94, 1063-1069.
g 2,4,6-Triphenylpyrylium.
Figure 2.
UV-visible spectra of zinc 2a-c2+ reduction products.
reduction in the pyrylium and aryl rings of the 4,2′- in
comparison to the 4,4′-regioisomer. This is supported by
decreases of the Mulliken atomic charges by 0.621 and 0.568
in the pyrylium and aryl rings upon reductions of 2a2+ and
12+, respectively. Finally, the Mulliken spin densities are
more localized in the pyrylium ring (0.63) of 2a+• but more
evenly distributed in the pyridinium (0.49) and pyrylium ring
(0.51) of 1+• which also explains the higher reactivity of
2a+• with oxygen (vide supra).
The B3LYP/6-31G(d) geometries of the 3 redox states
also establish the validity of the structural representations
shown in Scheme 2. For example, the inter-ring distance,
d42′, decreases from 1.499 to 1.442 to 1.392 Å in the series
2a2+, 2a+•, and 2a0 while the oxygen-carbon distance, d12,
increased from 1.355 to 1.368 to 1.385 Å. The inter-ring
dihedral angle also dramatically decreases from 78.45 to
35.41 to 19.31°, consistent with an increase in the π bond
order of bond 4-2′. The change in the hybridization at
nitrogen evident in the structural representations in Scheme
2 is also consistent with the decrease in the dihedral angle
>32′NCH3 in 2a2+ (178.63°), 2a+• (168.00°), and 2a0
(149.28°) and with the implied loss of aromaticity in the
pyrylium and pyridinium ring along this redox series.
Chemical reductions of 2a-c2+ with zinc dust in an
acetonitrile slurry resulted in disappearance of the pyrylogen
and appearance of long wavelength UV-vis bands as shown
in Figure 2. We assign these new species to the radical
cations, 2+•. Unfortunately, TD-DFT calculations with
B3LYP/6-31G(d) geometries of open shelled species that
form charge transfer excited states similar to 2+• are well-
known to be unrealiable and cannot be used to verify these
assignments.9 On the other hand, TD-DFT calculations with
the B3LYP/6-31G(d) model with closed shell species are
known to given excellent agreement with experimental
UV-visible spectra. Indeed, the calculated wavelengths of
the long wavelength absorbances of 2a2+ (442 nm), 2b2+
(518 nm), 2c2+ (430 nm) are in excellent agreement with
the observed values of 435 nm, 510 nm, and 410 nm,
respectively. Consequently, the very poor agreement between
the calculated λMAX for 2a0 (595 nm, f ) 0.0907; 467 nm, f
) 0.9597), 2b0 (572 nm, f ) 0.1008; 510 nm, f ) 0.3979),
and 2c0 (665 nm, f ) 0.1426; 535 nm, f ) 0.6284) and the
experimental values in Figure 2 argue against a two electron
chemical reduction and instead are consistent with our
assignment of the UV-vis spectra of the Zn reduction
products to the radical cations. In addition, treatment of both
Photophysical Behavior. The UV-visible spectra of the
pyrylogens and for 2,4,6-triphenylpyrylium tetrafluoroborate,
3, are shown in Figure 3. The UV-vis spectra of the 4,4′-,
12+, and 4,2′-, 2a2+, pyrylogens are very similar both
exhibiting 3 bands with similar relative intensities and
wavelengths. The oscillator strengths, however, are larger
in 12+ than in 2a2+ at the λMAX of all three bands. The long
wavelength bands in all the pyrylogens are also bathochromic
of the long wavelength band in 3 (i.e., λMAX 2b > 1 > 2a >
2c > 3).
All of the pyrylogens emitted both fluorescence and
phosphorescence at the wavelengths listed in Table 1. The
singlet energies were determined from the crossing points
of the fluorescence and normalized absorption spectra. The
triplet energies were estimated using the short wavelength
onset of the phosphorescence spectra. The fluorescence
quantum yields of the pyrylogens, ΦPY, were determined in
acetonitrile and compared to the fluorescence quantum yield
of 9,10-diphenyl anthracene ΦDPA using equation 1 in which
APY and ADPA are the absorbances at the excitation wave-
length, nACN and nCyclohexane are the refractive indicies of the
(8) (ii), (io), and (oo) refer to rotomeric isomers of 2b involving the
methoxy group. The o refers to the rotomer in which the methyl group
points out toward the pyridinium ring and i to the rotomer in which the
methyl group points in towards the pyrylium oxygen.
(9) (a) Zhang, D.; Telo, J. P.; Liao, C.; Hightower, S. E.; Clennan, E. L.
J. Phys. Chem. 2007, 111, 13567–13574. (b) Dreuw, A.; Head-Gordon, M.
J. Am. Chem. Soc. 2004, 126, 4007–4016.
(10) Clennan, E. L.; Liao, C. unpublished results.
Org. Lett., Vol. 11, No. 3, 2009
687

triplet based on the ability of oxygen to dramatically reduce
its lifetime and the ability of bromobutane to enhance its
intensity by approximately 30%. In addition, the intensities
of similar transients observed in the LFP of 2b2+ (appx. 625
nm; τ ) 37 ( 16 µs) and 2c2+ (530 nm; τ ) 25 ( 6 µs)
mirrored the intensities of their phosphorescence emissions.
Figure 3
.
UV-vis spectra [λMAX(ε)] for 1 (2.3 × 10-5 M), 2a (2.29
× 10-5 M), 2b (1.07 × 10-5 M), 2c (8.5 × 10-6 M), and 3 (1.25
× 10-5 M).
solvents, and FPY and FDPA are the integrated emission
areas.11 The fluorescence quantum yields decrease as the
singlet-triplet energy gap gets smaller consistent with an
increase in the rate of intersystem crossing. The phospho-
rescence spectra were collected in an HCl gas saturated
ethanol matrix at 77 K. Control experiments demonstrated
that the pyrylogens were stable under these conditions. The
relative quantum efficiencies estimated from the normalized
integrated intensities of the phosphorescence spectra (12+ 1.0;
2a2+ 0.54; 2b2+ 0.025; 2c2+ 0.94) are smaller for the 4,2′
than for the 4,4′ pyrylogen. The ability of the p-methoxy
substitutent in 2b2+ to reduce the intensity of phosphores-
cence was also noted previously in the 4,4′-regioisomer.2
Figure 4.
LFP Spectrum of 2a2+ 4.7 µs after irradiation at 355 nm
in 1,2-dichloroethane; (Inset) decay at 550 nm.
In conclusion, the 4,2′-pyrylogens all have low energy
absorption bands that will allow irradiations of many reaction
mixtures without concern about competitive absorption by
substrates. In addition, their ease of reduction coupled with
their excited-state energies provide potent oxidants that in
the case of the S1 state of 2c2+ is capable of removing an
electron from substrates with oxidation potentials as high as
3.0 eV. (reduction potentials: 12+ S1-2.73 eV, T1-2.51 eV;
2a2+ S1-2.71 eV, T1-2.51 eV; 2b2+ S1-2.18 eV, T1-2.04 eV;
2c2+ S1-3.00 eV, T1-2.65 eV).
2
ADPAFPY(nACN
)
ΦPY
)
ΦDPA
(1)
2
[
]
APYFDPA(nCyclohexane
)
Acknowledgment. We thank the National Science Foun-
dation for their generous support of this research.
Nanosecond laser flash photolysis (LFP) studies with 2a2+
in 1,2-dichloroethane resulted in the observation of a
prominent peak at 550 nm with a lifetime of 14 ( 2 µs as
shown in Figure 4. We have identified this transient as the
Supporting Information Available: Synthesis, compu-
tational data, X-ray, and ESR. This material is available free
of charge via the Internet at http://pubs.acs.org.
(11) Eaton, D. F. In Handbook of Organic Photochemistry; Scaiano,
J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 1, pp 231-239.
OL802763R
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Org. Lett., Vol. 11, No. 3, 2009