A new free and immobilized pyrylogen electron transfer sensitizer

Article abstract of DOI:10.1016/j.tetlet.2009.12.102

The synthesis, photophysical, and electrochemical characterization of N-allyl-2,6-diphenyl-4,4′-pyrylogen bis tetrafluoroborate is reported. The pyrylogen is reduced under dissolving metal reduction conditions with formation of the radical cation. In addi

Full text of DOI:10.1016/j.tetlet.2009.12.102

Tetrahedron Letters 51 (2010) 1249–1251
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new free and immobilized pyrylogen electron transfer sensitizer
b,
Tamer T. El-Idreesy a, , Edward L. Clennan
*
*
a Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
b Department of Chemistry, University of Wyoming, 1000 East University Avenue, Laramie, WY 82071, United States
a r t i c l e i n f o
a b s t r a c t
The synthesis, photophysical, and electrochemical characterization of N-allyl-2,6-diphenyl-4,40-pyrylo-
gen bis tetrafluoroborate is reported. The pyrylogen is reduced under dissolving metal reduction condi-
tions with formation of the radical cation. In addition, it can be copolymerized with styrene and
divinylbenzene to generate a polymer bound reagent that facilitates its separation from photochemically
induced electron-transfer reaction mixtures.
Article history:
Received 19 November 2009
Revised 17 December 2009
Accepted 17 December 2009
Available online 28 December 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Charge shift (CS) electron-transfer reactions that lead to forma-
tion of neutral radical/radical–cation pairs that lack a coulombic
barrier for diffusive separation have been employed for many years
to improve the efficiencies of photoinduced electron-transfer (PET)
reactions .1 Measurements of the quantum yields for the separa-
tion of the radical/radical–cation pair have shown that this strat-
egy is reasonably effective.2 However, there is room for
improvement and as a consequence pyrylogen, 12+, (Scheme 1)
has recently been introduced as a new charge shift electron-trans-
fer sensitizer.3 The charge shift in the critical electron-transfer step
with this PET sensitizer generates a repulsive radical–cation/radi-
cal–cation pair. The rapid eletrostatically assisted separation of
these radical cations competes with energy wasting return elec-
tron transfer and results in a more efficient ‘green’ photoinduced
electron-transfer (PET) reaction.4
Ph
O
Ph
O
2 BF
2 BF
4
4
a
N
N
R
2'
Ph
Ph
2+
2+
1
2
R = H
R = CH =CH
2+
3
2
Scheme 1. Pyrylogens.
N
O
N
NaH
O
O
In this Letter we report the synthesis of a new pyrylogen, 22+
,
+
and its characterization that provides additional insight into the
photochemical behavior of this new class of charge shift elec-
tron-transfer sensitizers. In addition, we report the synthesis of
the first polymer-bound pyrylogen, 32+, in order to take advantage
of all the desirable features, including enhanced stability of the
CH OH
3
Ph
Ph
CHO
5
N
Br
Br
CH CN
sensitizer,5 reported for other insoluble polymer-bound reagents/
Ac O/HBF
2
4
2+
2
7
dyes such as Rose Bengal,6 fullerene C60
,
and pyrylium salts.8
O
O
o
80 C/30 min
The synthesis of 22+ was accomplished by the three-step proce-
dure shown in Scheme 2. In this reaction two moles of acetophe-
none are allowed to react with pyridine-4-carboxaldehyde to
give a 1,5-diketone, 5. Allylation to give 6 followed by oxidative
ring closure generated the purified pyrylogen product in an overall
yield of 28%.
3
Ph COH
3
Ph
Ph
6
Scheme 2. Synthesis of pyrylogen 22+
.
The structure of pyrylogen 22+ was confirmed using standard
spectroscopic techniques. The electrospray mass spectrum (see
Supplementary data) exhibits a peak at 351.13 m/z consistent with
formation of the radical cation, a peak for the (22+ + methoxy ad-
duct) at 382.20 m/z, and a peak at 310.33 m/z for the loss of the
allylic cation. The 1H and 13C NMR spectra of both 12+ and 22+ in
CD3CN were significantly broadened. It is possible that CD3CN is
acting as a reducing agent to produce a small amount of paramag-
netic radical cation that is responsible for the broadening. The
* Corresponding authors. Tel.: +2 02 35676619 (T.T.E.); tel.: +1 307 766 6667; fax:
+1 307 766 2807 (E.L.C.).
E-mail addresses: tamertawhid@yahoo.com (T.T. El-Idreesy), clennane@uwyo.
edu (E.L. Clennan).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.12.102

1250
T. T. El-Idreesy, E. L. Clennan / Tetrahedron Letters 51 (2010) 1249–1251
Formation of 2+Å begins almost immediately upon addition of a
4
3
2
1
0
degassed solution of 22+ to zinc and the green color continues to
intensify to reach a maximum after approximately 16 min (Fig. 1
top). Remarkably, at this point the concentration of 2+Å begins to
decrease concomitantly with the apparent/possible increase of
the dication, 22+ concentration (Fig. 1 bottom). Oxidation of 2+Å
by adventitious oxygen to form superoxide can be ruled out as a
mechanism for this process since it is thermodynamically uphill
(Table 1) by more than 10 kcal/mol.10 A disproportionation mech-
anism to simultaneously form a reduced and oxidized product (22+
+ 20 ? 2 2+.) can also be eliminated by consideration of its electro-
chemical behavior. In particular, pyrylogen 22+ is reduced in two
chemically reversible and electrochemically quasireversible steps
to the radical cation and neutral redox partners (Table 1). The equi-
librium constant for disproportionation, Keq = [2+]2/([22+Å][20]),
which can be calculated11 from the difference between the first
and second reduction potentials of 22+ (log Keq = [0.17 ꢀ (ꢀ0.34)]/
0.059) is far too large (4.4 ꢁ 108) to account for the extent of disap-
pearance of 2+. or amount of 22+ formed (Fig. 1 bottom).
t = 0
t = 3
t = 6
t = 8
t = 10
t = 12
t = 14
t = 16
200
300
400
500
600
700
800
900
Wavelength (nm)
4
3
2
1
0
t = 18
t = 20
t = 22
t = 24
t = 26
t = 28
t = 30
t = 35
The radical cation, 2+Å, is unreactive with oxygen even at the
100 mV/s time scale of a cyclic voltammetry experiment. However,
the radical cation clearly does react with oxygen on the laboratory
time scale. Consequently, it is tempting to suggest that the forma-
tion of the dication, 22+, from the radical cation, 2+Å, occurs by reac-
tion with adventitious oxygen via the mechanism depicted in
Scheme 3. The fact that not all of 22+ is recovered reflects an equi-
librium established in the last step, or potentially competitive
reductive decompositions of the hydroperoxides.
200
300
400
500
600
700
800
900
Wavelength (nm)
Figure 1. UV–vis spectra of Zn reduction of 22+ in CH3CN. Top; Increase in
absorbance at 616 nm as a function of time (min). Bottom; Decrease in absorbance
at 616 nm as a function of time (min).
The UV–vis spectrum of 22+ (Fig. 1; t = 0) exhibits three absorp-
tion maxima at kMAX(e
) = 282 nm (40,000 Mꢀ1 cmꢀ1), 325 nm
ability of CH3CN to act as a reducing agent in reactions with tetra-
cyanoethylene and iodine has previously been reported.9 The elec-
tron transfer in these reactions is induced by irradiation into a
weak charge transfer band. Indeed, this conjecture is also sup-
ported in our case by the appearance of a small peak at 620 nm
in the UV–vis spectrum of an acetonitrile solution of 22+ that was
irradiated with either a conventional lamp (350 nm) or with a
Nd:YAG laser at 355 nm. We have verified that this peak is the rad-
ical cation by independently synthesizing both 1+Å and 2+ by Zn
metal reduction under a vacuum atmosphere in CH3CN. The solu-
tion of 1+Å and 2+Å are intense in color and absorb at 620 nm and
616 nm, respectively (Fig. 1). The behavior of 22+ is in sharp con-
trast to the laser flash photolysis of 12+ in the less polar 1,2-dichlo-
roethane that preferentially leads to the observation of a transient
absorbance at 570 nm that is quenched by oxygen and is character-
istic of the triplet.3 Finally, broadening of the NMR spectra is not
observed in CDCl3 in the presence of approximately 33% trifluoro-
acetic acid added to increase solubility.
(11,000 Mꢀ1 cmꢀ1), and 441 nm (23,000 Mꢀ1 cmꢀ1). Time depen-
dent density functional theory calculations on the B3LYP/6-
311+G(d) geometry minimum of 22+ reveals that the lowest energy
absorption band corresponds exclusively to the HOMO?LUMO
transition. This transition involves a significant charge transfer
from the pyrylium to the pyridinium ring (Fig. 2). Pyrylogen 22+
also fluoresces and phosphoresces allowing determination of
approximate singlet and triplet energies (Table 1). In addition, at
77 K the phosphorescence spectrum of 22+ exhibited a second peak
that could be eliminated at longer delay times indicative of delayed
fluorescence (See Supplementary data).
Two minimum energy conformations of 22+ can be located with
the B3LYP/6-311+G(d) computational model (Fig. 3). The two con-
formations differ primarily by the orientation of the allyl group. In
0
the lowest energy conformation the vinyl-Ca–N–C2 dihedral angle
is 46° and vinyl group is eclipsed with an allylic C–H bond. In the
Table 1
Photophysical and electrochemical data for pyrylogens 12+ and 22+
N
O
N
O
CH CN
12+
22+
3
O
2
or
2
E1/2(1)a
E1/2(2)a
0.17
0.17
ꢀ0.34
536
O
O
O
ꢀ0.35
b
kF
533
O
c
kP
sF
565
590
d
Ph
Ph Ph
N
Ph
3.42 0.23
35.6 0.2
59
e
sP
30.4
58
53
1
E(S1)f
E(T1)f
N
O
54
g
UF
0.18 0.01
0.33 0.09
2
a
b
c
d
e
f
or
+ HOO
2
In volts versus SCE.
O
In nanometers (nm) in CH3CN at 298 K.
In nanometer nm in EtOH/HCl(g) at 77 K.
In nanoseconds (ns).
In milliseconds (ms) at 77 K in EtOH/HCl(g).
In kcal/mol.
OH
OH
O
Ph
Ph Ph
O
Ph
g
Scheme 3. Potential mechanism for the formation of 22+ from 2+.
Fluorescence quantum yield in CH3CN.

T. T. El-Idreesy, E. L. Clennan / Tetrahedron Letters 51 (2010) 1249–1251
1251
meation chromatography (GPC) using polystyrene standards. The
polymer was found to have a weight average molecular weight
(MW) = 20,089 and
a
number average molecular weight
(Mn) = 9818 and hence a polydispersity (PDI) of 2.046.
In summary, we have reported the synthesis and the electro-
chemical and photophysical characteristics of a new pyrylogen;
the first that replaces the N-methyl group with another substitu-
ent. We have also demonstrated that this pyrylogen can be incor-
porated into a heterogeneous catalyst. The utility of this new
catalyst in electron-transfer reactions will be reported in the
future.
Figure 2. HOMO–LUMO transition in 22+
.
Acknowledgments
We thank the USA National Science Foundation (ELC) and the
Bibliotheca Alexandrina—Center for Special Studies and Programs
(BA/CSSP) under Grant No. BA2007/2008 (TTE) for their generous
support of this research. We also thank Will Welch for the compu-
tational results.
Supplementary data
Supplementary data (synthetic procedures, polymerization pro-
cedure, structural, photophysical, electrochemical and computa-
tional characterization) associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2009.12.102.
Figure 3. Two views of the B3LYP/6-311+G(d) minima for 22+
.
References and notes
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0
higher energy conformation (1.75 kcal/mol), the vinyl-Ca–N–C2
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