Pyrylogens: Synthesis, structural, electrochemical, and photophysical characterization of a new class of electron transfer sensitizers

Article abstract of DOI:10.1021/ja802343v

The synthesis and photophysical properties of a new series of dicationic electron transfer sensitizers have been reported. These new materials, pyrylogens, are hybrids of pyrylium cations and Viologen dications. Electron transfer reactions of neutral orga

Full text of DOI:10.1021/ja802343v

Published on Web 05/24/2008
Pyrylogens: Synthesis, Structural, Electrochemical, and Photophysical
Characterization of a New Class of Electron Transfer Sensitizers
Edward L. Clennan,* Chen Liao, and Erambo Ayokosok
Department of Chemistry, UniVersity of Wyoming, 1000 East UniVersity AVenue, Laramie, Wyoming 82071
Received March 31, 2008; E-mail: clennane@uwyo.edu
Scheme 1. Pyrylogens and the Repulsive CSP
The past 25 years has seen the development of a remarkable
array of photochemical electron transfer (PET) reactions with
significant synthetic potential.1 Unfortunately, this potential is often
not realized because return electron transfer (RET) limits the
efficiencies of these reactions and in the worst case scenarios allows
detrimental side reactions to dominate. Consequently, it is not
surprising that considerable effort has been expended to prevent
energy-wasting RET.2 These efforts include (1) thermodynamic
manipulations of the energetics of return electron transfer to put
the system deep in the Marcus inverted region where RET is too
slow to compete with the chemical reaction of interest;3 (2) the
use of steric effects in either the substrate or sensitizer to increase
substrate-sensitizer distance in the ion pair intermediate decreasing
the electronic coupling matrix element for RET;4 (3) the generation
of triplet ion pair intermediates in which RET is spin-forbidden;5
(4) amplification of the quantum yields by design of electron transfer
chain reactions;6 (5) the use of cosensitizers;2a and (6) the use of
charge-shift reactions that eliminate Coulombic attraction between
the reduced sensitizer and substrate radical cation thereby enhancing
their diffusive separation.2b
Scheme 2. Synthesis of Pyrylogens
Table 1. Photophysical and Electrochemical Data for 1a-d and 2+
In this Communication we report the synthesis and properties
of a new class of sensitizers, pyrylogens 1a-d (Scheme 1), that
are formally hybrids of pyrylium cations and viologens. These
compounds were fabricated to extend the charge shift concept from
cationic to dicationic sensitizers. The diaryl-pyrylium chromophores
in the pyrylogens impart desirable optical properties while the
pyridinium ring provides a residual positive charge on the sensitizer
in the charge shift product (CSP) that is designed to repel the
substrate radical cation and competitively inhibit RET.
1a
1b
1c
1d
2+
E1/2(1)a 0.17
0.19
0.05
-0.42
0.37
-0.35
-1.53
465
E1/2(2)a -0.35
-0.35
-0.27
b
λF
λP
τF
τP
533
565
556
588
659
665
494
546
c
520
d
e
3.42 ( 0.23
35.6 ( 0.2 38.3 ( 1
2.1 ( 0.21 4.1(2.9)j
25.4 ( 0.6 33.9 ( 0.2 225 ( 3
E(S1)f
E(T1)f
59
54
56
53
48
48
h
63
57
65i
53i
g
ΦF
0.18 ( 0.01 0.43 ( 0.01
0.03 ( 0.02
0.24 ( 0.01 0.60i
The key step in the synthesis of the pyrylogens7 (Scheme 2) is
oxidative cyclization of a 1,5-dione patterned after the most popular
method to generate triarylpyrylium salts.8 Triphenyl carbenium ion9
is preferred to benzalacetophenone as a hydride acceptor because
a small amount of retro-Michael addition in the 1,5-diketone can
lead to formation of trace amounts of a triarylpyrylium cation
contaminant. The structures of the pyrylogens were confirmed by
standard structural characterization methods (see Supporting In-
formation) and in the case of 1a by metathesis with KB(C6F6)4 to
form crystals suitable for X-ray diffraction. In the solid state the
pyrylogen is sandwiched in between the two counterions and is
characterized by rotation of the pyridinium ring 53° out of the plane
of the pyrylium core.
The pyrylogens, 1a-d, are reduced by cyclic voltammetry (CV)
in two one-electron chemically reversible and electrochemically
quasireversible steps (Table 1). The reduction potentials for
formation of both the radical cation (slope ) 0.34; R2 ) 0.9988)
and the neutral (slope ) 0.12; R2 ) 0.97) redox partners are linearly
related to the Hammett substituent constant σ+. Replacement of
the 4-phenyl ring in 2,4,6-triphenylpyrylium tetrafluoroborate, 2+,
with the methyl-pyridinium ring in 1a increases the ease of
reduction by over 500 mV. The pyrylogen radical cations, in stark
ΦT
a In volts versus SCE. b In nanometers (nm) at 298K. c In nm in
EtOH/HCl(g) at 77K. d In nanoseconds (ns). e In milliseconds (ms) at
77K in EtOH/HCl(g). f In kcal/mol. g Fluorescence quantum yield in
CH3CN. h Too small to measure. i Miranda, M. A.; Garc´ıa, H. Chem.
ReV. 1994, 94, 1063-1069. j Haucke, G.; Czerney, P.; Cebulla, F.
Absorption and Fluorescence of Pyrylium Salts. Ber. Bunsenges. Phys.
Chem. 1992, 96, 880-886. CH3CN(CH2Cl2).
contrast to 2•, do not react with oxygen even at 50 mV/sec scan
rates. We attribute this reluctance to react with oxygen to the greater
positive charge in the pyrylogen radical cations and to their more
delocalized spin density in comparison to 2•. Neither 2• nor 1+•a-d
are thermodynamically capable of producing superoxide. Further-
more, irradiations of pyrylogens 1a-d at 355 nm, where all the
pyrylogens absorb (vide infra) did not produce emission at 1270
nm consistent with their inability to sensitize formation of singlet
oxygen (1∆g).
The UV-vis spectra for the pyrylogens and 2+ are compared in
Figure 1. The two lowest energy bands in 2+ at 401 and 355 nm
have been interpreted as two perpendicular chromophores polarized
in the X and Y directions, respectively.10 This assignment is
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7552 J. AM. CHEM. SOC. 2008, 130, 7552–7553
10.1021/ja802343v CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. A comparison of the UV-vis spectrum of 1a and its radical
cation.
Figure 1. UV-visible spectra of 1a-d and 2 in CH3CN.
confirmed by time dependent DFT calculations on the B3LYP/6-
31G(d) structure of 2+ as shown in the Supporting Information.
The UV-vis spectra of 1a-d are qualitatively similar (Figure 1)
but the Y-band, which is a charge-transfer band from the 4-aryl
ring to the pyrylium core, is missing in the pyrylogens as a result
of the electron poor character of the 4-pyridinium ring and has been
replaced by a weaker intensity band that is essentially a charge
shift from the 2,6-aryl rings to the pyridinium ring.
method, which only requires experimental ratios ΦT/ΦT0 from LFP
and Φf0/Φf from Stern Volmer studies with 1-bromobutane, gives
a value of 0.03 ( 0.02 for ΦT (Table 1).
Finally, we have also demonstrated that these pyrylogens act as
sensitizers in the cleavage of r-1-, c-2-, t-3-, t-4-tetraphenyl-
cyclobutane to form trans-stilbene.14 Work to establish the mech-
anism and to maximize and measure absolute efficiencies of these
reactions are in progress.
0
All of the pyrylogens also fluoresce and phosphoresce at the
wavelengths shown in Table 1. The significantly smaller quantum
yields of fluorescence for the pyrylogens in comparison to 2+ is
consistent with a previous suggestion that the Y chromophore,
which is missing in the pyrylogens, is the predominant fluorophore
in 2+.11 The crossing point of the normalized absorption and
fluorescence spectra were used to calculate the S1 energies and the
onset of the phosphorescence spectra were used to estimate the T1
energies. These energies coupled with the reduction potentials
suggest that the ionization potentials of the S1 states increase in
the order 1c (2.13 eV) < 2+ (2.47 eV) < 1b (2.62 eV) < 1a (2.73
eV) < 1d (3.10 eV) and the T1 states in the order 2+ (1.95 eV) <
1c (2.13 eV) < 1b (2.49 eV) < 1a (2.51 eV) < 1d (2.84 eV).12
These data suggest that S1 and T1 of all the pyrylogens, with the
exception of electron rich S1-1c, are more potent oxidants than the
popular pyrylium sensitizer, 2+.
Laser flash photolysis (LFP) of 1a in 1,2-dichloroethane resulted
in observation of two peaks at 370 and 560 nm (Supporting
Information). Both peaks decay at the same rate suggesting that
they arise from a common transient species. We have identified
this transient as the triplet based upon the ability of oxygen to
quench both the 370 and 560 nm peaks and on our previous
observation that the pyrylogen radical cations do not react with
oxygen even on the much longer time scale of a CV experiment.
In addition, an independently generated radical cation of 1a absorbs
strongly at 614 nm (Figure 2).
Acknowledgment. We thank the National Science Foundation
for their generous support of this research and Peter Ogilby and
Mette Johnsen (Aarhus University Demark) for τF measurements.
Supporting Information Available: Synthesis of 1a-d and X-ray
data for 1a·2B(C6F5)4. This material is available free of charge via
the Internet at http://pubs.acs.org.
References
(1) Mangion, D.; Kendall, J.; Arnold, D. R. Org. Lett. 2001, 3, 45–48.
(2) (a) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem. Soc. 1990,
112, 4290–4301. (b) Gould, I. R.; Moser, J. E.; Armitage, B.; Farid, S.
Res. Chem. Intermed. 1995, 21, 793–806. (c) Gould, I. R.; Moser, J. E.;
Ege, D.; Farid, S. J. Am. Chem. Soc. 1988, 110, 1991–1993.
(3) Gould, I. R.; Noukakis, D.; Gomez-Jahn, L.; Goodman, J. L.; Farid, S.
J. Am. Chem. Soc. 1993, 115, 4405–4406.
(4) Gould, I. R.; Farid, S. J. Phys. Chem. 1993, 97, 13067–13072.
(5) Roth, H. D. Pure Appl. Chem. 2005, 77, 1075–1085.
(6) Adam, W.; Asensio, G.; Curci, R.; Marco, J. A.; Gonzalez-Nunez, M. E.
Tetrahedron Lett. 1992, 33, 5833–5836.
(7) For the synthesis of 1a see: Katritzky, A. R.; Adamson, J.; Elisseou, E. M.;
Musumarra, G.; Patel, R. C.; Sakizadeh, K.; Yeung, W. K. J. Chem. Soc.,
Perkin Trans. 2 1982, 1041–1048.
(8) Miranda, M. A.; Garc´ıa, H. Chem. ReV. 1994, 94, 1063–1069.
(9) Branchi, B.; Doddi, G.; Ercolani, G. J. Org. Chem. 2005, 70, 6422–6428.
(10) Balaban, A. T.; Fischer, G. W.; Dinculescu, A.; Koblik, A. V.; Dorofeendo,
G. N.; Mezheritskii, V. V.; Schroth, W. Pyrylium Salts: Synthesis, Reaction,
and Physical Properties In AdVances in Heterocyclic Chemistry; Katritsky,
A. R., Ed.; Academic Press: New York, 1982; Suppl. 2.
(11) Montes-Navajas, P.; Teruel, L.; Corma, A.; Garcia, H. Chem. Eur. J. 2008,
14, 1762–1768.
(12) The IPs are given by [E(S1 or T1) kcal/mol]/23.06 + E1/2(1).
(13) Horrocks, A. R.; Medinger, T.; Wilkinson, F. Photochem. Photobiol. 1967,
6, 21–28.
(14) Miranda, M. A.; Izquierdo, M. A.; Galindo, F. J. Org. Chem. 2002, 67,
4138–4142.
The triplet-triplet absorbance spectrum of 1a is dramatically
enhanced and its fluorescence quenched by the addition of 1-bro-
mobutane. This allowed measurement of the quantum yield for
triplet formation using the method of Wilkinson.13 This accurate
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