Synthesis, structural, electrochemical, and photophysical properties of 4,2′-thiopyrylogens - A novel class of sensitizers for photoinduced electron transfer reactions

Article abstract of DOI:10.1002/poc.1697

The first three examples of the thioanalog of the 4,2′-pyrylogen ring system are reported. The influence of the sulfur atom on the structural, electrochemical, and photophysical behavior of this ring system is discussed. In addition, these 4,2′-thiopyrylo

Full text of DOI:10.1002/poc.1697

Research Article
Received: 9 September 2009,
Revised: 2 December 2009,
Accepted: 29 January 2010,
Published online 4 June 2010 in Wiley Online Library: 2011
(wileyonlinelibrary.com) DOI 10.1002/poc.1697
Synthesis, structural, electrochemical, and
photophysical properties of 4,20-
thiopyrylogens – a novel class of sensitizers
for photoinduced electron transfer reactions
Ajaya Kumar Sankara Warriera and Edward L. Clennana
*
The first three examples of the thioanalog of the 4,2(-pyrylogen ring system are reported. The influence of the sulfur
atom on the structural, electrochemical, and photophysical behavior of this ring system is discussed. In addition, these
4,2(-thiopyrylogens are compared to their previously reported 4,4(-isomers and their 4,4(- and 4,2(-oxygen analogs.
Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper
Keywords: electron transfer; cyclic voltammetry; pyrylium; pyrylogen; sensitizer
refractive,[17] and electrochromic[18] materials, would also be
exhibited by the pyrylogens. To a large extent our hopes were
INTRODUCTION
It is remarkable that despite the fact that the total mass of the
electrons is dwarfed by the mass of the nuclei, the loss or gain of a
single electron frequently leads to skeletal rearrangements in
organic substrates.[1] These reactions are often faster or lack
counterparts in the neutral precursors and are, as a consequence,
valuable additions to the synthetic chemists’ repertoire. Photo-
induced electron transfer (PET) has emerged as a very versatile
method to conduct these reactions.[2] However, PET reactions
suffer from inefficiency associated with return electron transfer
(RET) in the initially formed ion pair.[3] In order to combat this
inefficiency we recently introduced the use of pyrylogens, 12R, as
a new class of PET catalysts that generate repulsive radical-cation/
realized with the previously reported 2,6-diaryl-4,40-pyrylogens;[4]
however, these compounds do have some practical limitations
including a propensity to hydrolyze. In the search for even more
robust sensitizers, we were attracted to reports that thiopyrylium
salts are more stable than their pyrylium analogs under a variety
of conditions.[19] In addition, their enhanced spin orbit coupling
that will lead to triplet ion pairs in which RET is spin forbidden,
coupled with the expectation of their potent oxidizing
capability,[20,21] suggested thiopyrylogens as worthy synthetic
targets. We report here the first examples of the 4,20-
thiopyrylogen ring system, 4a-c2R, and compare their photo-
physical and electrochemical properties to the previously
radical-cation pairs whose rapid diffusive separation, kdiff
,
reported N-methyl-2,6-diphenyl-4,40-thiopyrylogen, 52R
methyl-2,6-diphenyl-4,20-pyrylogen, 62R
and N-methyl-2,6-
diphenyl-4,40-pyrylogen, 72R
,
N-
competitively inhibits RET.[4]
,
.
These new materials were designed in anticipation that they
would exhibit the favorable optical properties[5–7] of the better-
known and more thoroughly investigated pyrylium salts such as
triphenyl-pyrylium, 2R, and -thiopyrylium, 3R, tetrafluorobo-
rates.[7,8] In addition, we hoped that the robust character of the
pyrylium salts that allow their use as photoinitiators for
polymerizations,[9] as laser dyes,[10,11] as sensitizers in photo-
dynamic therapy,[12,13] as protein stains,[14] as photoconductive
elements,[15] and as components of photofunctional,[16] photo-
*
Correspondence to: E. L. Clennan, Department of Chemistry, University of
Wyoming, 1000 East University Avenue, Laramie, WY 82071, USA.
E-mail: clennane@uwyo.edu
a
A. K. S. Warrier, E. L. Clennan
Department of Chemistry, University of Wyoming, 1000 East University
Avenue, Laramie, Wyoming 82071, USA
J. Phys. Org. Chem. 2011, 24 22–28
Copyright ß 2010 John Wiley & Sons, Ltd.

A NOVEL CLASS OF ELECTRON TRANSFER SENSITIZERS
RESULTS AND DISCUSSION
The N-methyl-2,6-diaryl-4,20-thiopyrylogens, 4a–c2R, were made
as depicted in Scheme 1. The key step in the synthesis is the
oxidative cyclization of a 1,5-diketone in the presence of H2S.
The sequence of microscopic steps is not clear; however, hydride
abstraction by in situ generated triphenylcarbenium ion,
nucleophilic addition of a sulfur nucleophile to a carbonyl group,
cyclization to form a sulfur heterocycle, and loss of H2X (X ¼ S or
O) are all involved in this remarkable transformation. The
structures of the new thiopyrylogens and their 1,5-diketone
1
precursors (Scheme 1) are entirely consistent with their H and
13C NMR, electrospray ionization mass spectrometry, and UV–
visible spectroscopy data.
The UV–visible spectrum of 4a2R is compared to the UV–visible
spectra of its 4,40-isomer and its 4,40- and 4,20-oxygen analogs as
shown in Fig. 1. The wavelengths and extinction coefficients of
the lowest energy bands at 432 nm (e ¼ 22 600 Mꢀ1 cmꢀ1) for
4a2R, at 435 nm (e ¼ 15 600 Mꢀ1 cmꢀ1) for 52R, at 435 nm
Figure 1. A comparison of UV–Visible spectra of several pyrylogen and
thiopyrylogen dications in CH3CN.
(e ¼ 23 000 Mꢀ1 cmꢀ1
)
for
62R
,
and
at
441 nm
(e ¼ 31 000 Mꢀ1 cmꢀ1) for 72R are remarkably similar. Time
dependent density functional theory calculations on the
B3LYP/6-31G(d) geometries of these pyrylogens and thiopyrylo-
gens indicate that these low energy bands correspond in large
part to the HOMO!LUMO transition. In all of these cases, this
transition involves a significant charge transfer from a 2,6-
diphenylpyrylium chromophore to the pyridinium ring as
depicted in Fig. 2.
The B3LYP/6-31G(d) energies indicate that the 4,40-isomers in
both the pyrylogens and thiopyrylogens are approximately
8 kcal molꢀ1 more stable than their 4,20-isomers as shown in
Table 1. We attribute the bulk of this energy difference to a steric
interaction between the 2-N-methyl group and the pyrylium ring.
This interaction is undoubtedly also responsible for the
0
0
significantly larger inter-ring dihedral angle, >C3C4C02 N1 and
0
0
0
>C3C4C4 C3 , in the 4,2 - in comparison to their 4,4 -isomers,
respectively (74.38 and 78.48 in 4a2R and 62R versus 43.38 and
41.58 in 52R and 72R, respectively; Table 1). The 2,6-phenyl rings
are also significantly less co-planar with the pyrylium rings in the
thiopyrylogens (ꢁ29–328) than in the pyrylogens (ꢁ12–168). Also
˚
of note are the longer C—S (1.74 A) than C—O bond lengths
˚
(1.35 A) and smaller C—S—C (ꢁ1068) than C—O—C bond angles
(ꢁ1258).
Scheme 1.
J. Phys. Org. Chem. 2011, 24 22–28
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A. K. S. WARRIER AND E. L. CLENNAN
The fluorescence (lF) and phosphorescence (lP) maxima, the
phosphorescence lifetimes (tP), the quantum yields of fluor-
escence (FF), the Stokes shifts, and the singlet [E(S1)] and triplet
energies [E(T1)], of 4a–c2R, 52R, 62R, and 72R are given in Table 2.
The fluorescence quantum yields are slightly smaller for the
thiopyrylogens in comparison to the pyrylogens (compare 4a2R
to 62R and 52R to 72R) as anticipated for competitive heavy atom
enhanced inter-system crossing to the triplet manifold. Also of
note is the fact that the Stokes shifts are smaller for the 4,20- in
comparison to the 4,40-regioisomers for both pyrylogens and
thiopyrylogens (compare 4a2R to 52R and 62R to 72R). We
attribute the larger Stoke shifts in the 4,40-regioisomers to a
significant geometry change to a coplanar pyrylium ring-
pyridinium ring S1 excited state. A similar change in geometry
in the S1 excited states of the 4,20-regioisomers is sterically
precluded by the 20-N-methyl group. The substitution of sulfur for
oxygen has little if any effect on either the singlet or triplet
energies. In addition, these energies are also nearly identical in
the 4,20- and 4,40-regioisomers. On the other hand, introduction
of a methoxy group into the 2,6-phenyl rings significantly lowers
both the singlet and triplet energies (compare 4a2R to 4c2R
)
consistent with the well-known propensity of electron donating
groups to increase HOMO energies to a greater extent than
LUMO energies.
To further characterize the triplet excited state responsible for
the phosphorescence we have also examined 4a2R by laser
flash photolysis. In these studies, 0.01% tetrafluoroboric acid
was added to the acetonitrile solution of 4a2R in order to inhibit
a slow hydrolysis that complicated the analysis. Irradiation of
this sample with 7 mJ 355 nm pulses from a Nd:YAG laser
Figure 2. HOMO and LUMO orbitals for pyrylogens 4a2R, 52R, 62R, and
72R calculated with the B3LYP/6-31G(d) model.
generated
a transient characterized by a broad diffuse
absorption from 470–570 nm (Fig. 3). This transient has a
lifetime of approximately 44 ꢂ 4 ms as determined by a first-
order fit of the decay of this transient at 560 nm (inset in Fig. 3).
We assign this species to the triplet based upon the fact that it
was completely quenched by the addition of oxygen and the
observation that the radical cation is inert to oxygen in this
timeframe (vide infra).
Table 1. B3LYP/6-31G(d) structural parameters for 4,40- and
4,20-pyrylogens and thiopyrylogens
The 4,20-thiopyrylogens 4a–c2R like the previously synthesized
pyrylogens are also characterized by chemically reversible and
electrochemically quasireversible reductions to both radical
cation and neutral redox partners (Scheme 2). The thiopyrylogens
are only slightly easier to reduce than their pyrylogen homologs
(compare 4a2R to 62R and 52R to 72R in Table 2) and the 4,40- are
only slightly easier to reduce than their 4,20-isomers (compare
52R to 4a2R and 72R to 62R). The similarities in the values of the
dication ! radical-cation and radical-cation ! neutral redox
couples for the 4,20- and 4,40-isomers are consistent with the
nearly identical calculated B3LYP/6-31G(d) energy differences
4a2R
52R
62R
72R
E(kcal molꢀ1)a rel.
d12
7.80b
˚
1.741 A
0c
1.739 A
8.11d
˚
1.355 A
0e
1.354 A
˚
˚
˚
1.491 A
˚
1.487 A
0
d44
d42
—
1.501 A
˚
1.369 A
—
1.499 A
˚
1.369 A
˚
˚
0
—
1.355 A
—
1.355 A
˚
˚
0
0
d1 2
>C2O(S)1C6
106.148 105.878 125.258 124.848
between the 4,20- and 4,40-dication, E(4a2R/52R) ¼ 7.8 kcal molꢀ1
,
0
0
>C3C4C4 C3
—
43.328
—
31.508
32.098
—
41.488
—
15.788
15.368
radical cation E(4aR./5R.) ¼ 8.0 kcal molꢀ1, and neutral isomers
E(4a8/58) ¼ 8.3 kcal molꢀ1 (Table 3).
0
0
>C3C4C2 N1
74.348
30.518
29.508
78.458
13.048
12.888
f
>O(S)1C2CaCb
>O(S)1C6Ca Cb
0f
The 4,20-radical cation 4aR. was also generated by chemical
reduction with zinc dust and observed directly by UV–Vis
spectroscopy (Fig. 4). The spectrum of the reduced product is
characterized by a broad absorbance centered at 620 nm that
we assigned to the radical cation. This assignment was verified
by the observation of a single Electron Spin Resonance (ESR)
line for the reduced product at g ¼ 2.00559. Parenthetically, it
also provides confirmation of the assignment of the transient
observed by laser flash photolysis to the triplet rather than the
radical cation.
0
a a comparison of the 4,20- and 4,40-isomer relative ZPE cor-
rected energies.
b 1339.926323 Hartrees.
c 1339.938753 Hartrees.
d 016.958342 Hartrees.
e 1016.971268 Hartrees.
f a, a0, b, b0 refer to the carbons in the aryl rings attached to the
pyrylium ion ring and ortho to the pyrylium ring, respectively.
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A NOVEL CLASS OF ELECTRON TRANSFER SENSITIZERS
Table 2. Photophysical and electrochemical properties of 4a–c2R, 52R, 62R, and 72R
4a2R
4b2R
4c2R
52R
62R
72R
E1/2(1)a
0.12
ꢀ0.42
530
576
—
39.0 ꢂ 0.01
0.24 ꢂ 0.05
59
0.17
ꢀ0.38
555
582
—
32.0 ꢂ 0.02
0.29 ꢂ 0.06
57
0.01
ꢀ0.45
644
651
—
0.19
ꢀ0.32
547
581
—
36.5 ꢂ 0.05
0.13 ꢂ 0.01
58
0.06
ꢀ0.41
511
567
—
39.5 ꢂ 2
0.54 ꢂ 0.01
61
0.17
ꢀ0.35
E
lF
lP
1/2(2)a
b
533
c
565
d
tF
3.42 ꢂ 0.23
35.6 ꢂ 0.2
0.18 ꢂ 0.01
59
e
tP
FF
34.9 ꢂ 0.05
f
E(S1)g
49
49
122
E(T1)g
54
98
55
110
55
112
54
76
54
92
Stoke Shifth
lMAX(e)
278(26 600)
432(22 600)
—
286(18 900)
445(17 500)
—
308(20 200)
411(4960)
522(23 900)
278(33 100)
435(15 600)
—
292(23 300)
323(8200)
435(23 000)
281(45 000)
327(16 000)
441(31 000)
a In volts vs. SCE.
b In nm at 298 K.
c In nm in EtOH/HCl(g) at 77 K.
d In nanoseconds (ns).
e In milliseconds (ms) at 77 K in EtOH/HCl(g).
f In kcal molꢀ1
.
g In kcal molꢀ1
h In nm.
.
The increase in the p-bond order of the bond connecting the
pyrylium and pyridinium rings in the series dication ! radical
cation ! neutral as suggested in Scheme 2 is supported by
B3LYP/6-31G(d) calculations that show a decrease in bond length
0
˚
˚
˚
in this series for both the 4,2 - (1.501 A ! 1.451 A ! 1.401 A) and
0
˚
˚
˚
4,4 -thiopyrylogens (1.491 A ! 1.442 A ! 1.398 A). Despite the
very similar changes in this bond length, both the 4,40-radical
cation and 4,40-neutral thiopyrylogens are significantly more
planar than their 4,20-isomers. For example, the pyrylium and
pyridinium rings in the neutral 4,40-thiopyrylogen, 58, are nearly
0
0
coplanar (>C3C4C4 C3 ¼ 1.88) but are significantly twisted in the
0
0
0
4,2 -isomer, 4a8 (>C3C4C2 N1 ¼ 24.178).
The anticipated improved hydrolytic stability of 4a2R was
Figure 3. Absorption spectra of triplet 4a2R formed by laser flash
photolysis in CH3CN/0.01% HBF4. Inset: Decay of triplet monitored at
560 nm.
verified by qualitatively comparing it to its oxygen analog, 62R
,
using UV spectroscopy. After 20 min in identical batches of wet
acetonitrile the absorbance of 62R decreased by 43% while the
absorbance of 4a2R decreased by less than 2%.
Scheme 2.
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A. K. S. WARRIER AND E. L. CLENNAN
Table 3. B3LYP/6-31G(d) structural parameters comparison for 4,40- and 4,20-thiopyrylogen redox partners
52R
4a2R
5R
4aR.
58
4a8
E(kcal/mol)a
d12
0b
1.739 A
7.8c
1.741 A
0d
1.755 A
8.3e
1.761 A
0f
1.7878
8.0g
1.7918
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
0
44 /d42
0
d
1.491 A
1.501 A
1.442 A
1.451 A
1.398 A
1.401 A
˚
1.355 A
˚
1.369 A
˚
1.368 A
˚
1.390 A
˚
1.384 A
˚
1.424 A
0
0
d1 2
>C2O(S)1C6
105.878
120.208
43.328
31.508
106.148
120.758
74.34
103.388
118.648
15.948
38.388
103.488
121.018
40.888
37.868
101.468
117.148
1.88
101.268
120.878
24.178
37.718
0
0
0
>C2 N1 C6
>C3C4C4 C3 />C3C4C2 N1
>O(S)1C2CaCb
0
0
0
0
h
29.508
35.818
a ZPE corrected relative energies.
b 1339.938753 Hartrees.
c 1339.926323 Hartrees.
d 1340.254178 Hartrees.
e 1340.240914 Hartrees.
f 1340.435238 Hartrees.
g 1340.435238 Hartrees.
h a, b refer to the carbons in the aryl rings attached to the pyrylium ion ring and ortho to the pyrylium ring, respectively.
grade solvents were used for photophysical studies without
further purification except in the case of acetonitrile, which was
further purified by distillation over calcium hydride and stored
˚
over 3 A molecular sieves. Hydrogen chloride gas for making
ethanol/HCl glass matrices was prepared by reacting concen-
trated sulfuric acid and sodium chloride. Melting points were
taken using a Thomas Hoover UniMelt apparatus and 1H and 13
C
NMRs were recorded on a Bruker 400 MHz spectrometer. All NMR
spectra were collected using TMS as an internal reference. UV–
Visible absorption spectra were collected on an Agilent 8453
spectrometer and emission spectra on a Cary Eclipse Fluor-
escence spectrometer. Laser flash photolysis experiments were
conducted with a Luzchem instrument coupled to a Quanta-Ray
10 MHz pulsed Nd:YAG laser. IR spectra were obtained on a
Nicolet NEXUS 470 FT-IR spectrometer. ESI-MS data were
collected with a Thermo-Finnigan LCQ mass spectrometer and
oxidation potentials measure by cyclic voltametry (CV) using a CH
Instruments CHI 600 potentiometer.
.
Figure 4. A comparison of the absorption spectra of 4a2R and 4aR in
CH3CN.
N-Methyl-2,6-diphenyl-4,20-thiopyrylogen bis-tetrafluoroborate,
4a2R
CONCLUSION
N-Methyl-1,5-diphenyl-3-(2-pyrydyl)pentanedione tetrafluorobo-
rate (465 mg, 1.08 mmol) and triphenylmethanol (360 mg,
1.38 mmol) were added to acetic anhydride (3 ml) and H2S(g)
was bubbled through the mixture while it was heated to 708C for
3 h. The reaction mixture was cooled to room temperature while
maintaining a constant flow of H2S(g) and tetrafluoroboric acid
(48% in water, 900 ml) was added dropwise. This mixture was
heated to 70 8C for 5 h while maintaining a steady stream of
H2S(g) and for 3 h without a flow of H2S(g). The reaction mixture
was then cooled to room temperature and anhydrous ether
added to induce the formation of a yellow precipitate. The
product was recrystallized from an acetonitrile/acetic acid
mixture to give 324 mg (58%). mp 259–260 8C (decomposes).
IR (KBr pellet) 1624, 1598,1475, 1408, 1055 cmꢀ1. ESI-MS found
341.2; calculated for C23H19NS2þ. 341.12. 1H NMR (400 MHz,
CD3CN) d 8.97 (d, 6.2 Hz, 1H), 8.92 (s, 2H), 8.76 (t, 8.0 Hz, 1H), 8.29 (t,
7.1 Hz, 1H), 8.20 (d, 7.9 Hz, 1H), 8.12 (d, 7.4 Hz, 4H), 7.88 (t, 7.5 Hz,
A new series of pyrylogens have been synthesized and their
photophysical and electrochemical behavior have been
examined. The photophysical and electrochemical properties of
these new 4,20-thiopyrylogens are remarkably similar to the
previouslystudiedpyrylogens;however, theirimprovedhydrolytic
stability offers a clear advantage as electron transfer sensitizers. In
addition, they are easily made with readily available materials and
can be purified and handled with no special precautions.
EXPERIMENTAL
General methods
The syntheses of compounds were carried out using commer-
cially available reagents without purification. Spectroscopic
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A NOVEL CLASS OF ELECTRON TRANSFER SENSITIZERS
2H), 7.79 (t, 7.7 Hz, 4H), 4.27 (s, 3H). 13C NMR (100 MHz, CD3CN) d
174.1, 151.4, 151.2, 148.8, 147.8, 136.07, 134.58, 133.54, 131.18,
130.63, 130.1, 49.0.
C23H17Cl2NS2þ 409.04. 1H NMR (400 MHz, CD3CN) d 8.97 (d, 6.0 Hz,
1H), 8.92 (s, 2H), 8.76 (t, 7.9 Hz, 1H), 8.27 (t, 6.8 Hz, 1H), 8.20 (d,
7.9 Hz, 1H), 8.09 (d, 8.5 Hz, 4H), 7.79 (d, 8.5 Hz, 4H), 4.26 (s, 3H). 13
C
NMR (100 MHz, CD3CN) d 172.7, 151.7, 151.1, 148.9, 147.9, 142.5,
133.9, 133.2, 132.0, 131.7, 131.3, 130.8, 49.04.
1,5-Di-p-chlorophenyl-3-(2-pyridyl) pentane-1,5-dione
4-Chloroacetophenone (3.25 g, 21 mmol) was added to an ice-
cold mixture of methanol (15 ml) and NaH (150 mg). This mixture
was warmed and allowed to stir at room temperature for 1 h and
then cooled back to 0–10 8C. Pyridine-2-carboxaldehyde (1.0 g,
9.34 mmol) was added dropwise while maintaining the tempera-
ture below 10 8C. This mixture was stirred at 0–10 8C for 5 h and
allowed to warm and stir at room temperature overnight. Water
was then added to generate a turbid solution that was extracted
with dichloromethane. The organic layer was dried (MgSO4),
filtered, and the solvent removed. A diethylether hexane mixture
was then added to the resulting oil to induce crystallization and
the product recrystallized from ethanol/water to give 1.9 g (52%)
of the pentanedione as colorless crystals, mp 104–105 8C. IR (KBr
Pellet), 2901, 1687, 1588, 1400, 1207, 1085, 991, 820, 525 cmꢀ1. 1H
NMR (400 MHz, CDCl3) d 8.46 (d, J ¼ 4.2 Hz, 1H), 7.88 (d, J ¼ 8.6 Hz,
4H), 7.57 (t, J ¼ 7.6 Hz, 1H), 7.39 (d, J ¼ 8.6 Hz, 4H), 7.35 (d, 7.8 Hz,
1H), 7.07 (t, J ¼ 6.2 Hz, 1H), 4.2 (tt, J ¼ 6.6 Hz, 7.0 Hz, 1H), 3.60 (dd,
J ¼ 7.6 Hz, 17.3 Hz, 2H), 3.41 (dd, J ¼ 6.0 Hz, 17.3 Hz, 2H). 13C NMR
(100 MHz, CDCl3) d 197.3,162.5, 149.2, 139.5, 136.4, 135.2, 129.5,
128.8, 124.0, 121.7, 43.40, 38.1.
N-Methyl-2,6-di-p-methoxyphenyl-4,20-thiopyrylogen bis-
tetrafluoroborate 4c2R
3-(1-Methylpyridinium-2-yl)-1,5-di-p-methoxyphenylpentane-1,5-
dione (189 mg, 0.385 mmol) and triphenylmethanol (99 mg,
0.41 mmol) were added to acetic anhydride (1.5 ml) and H2S(g)
was bubbled through this mixture while heating to 70 8C and
stirring for 3 h. HBF4 (48% in water, 250 ml) was added at room
temperature and the reaction mixture stirred for 45 min, and then
heated to 70 8C, and stirred for an additional 2.5 h. The supply of
H2S(g) was then shut off and the reaction mixture stirred for 3 h.
Diethylether was then added to the mixture at room temperature
to induce the crystallization of the crude yellow product. The
product was recrystallized from acetonitrile/acetic acid to give
100 mg of dark purple crystals (45%). mp 244–245 8C (decom-
poses). IR (KBr Pellet) 3107, 1602, 1510,1482, 1400, 1252, 1182,
1056, 1014, 832, 560 cmꢀ1. ESI-MS found 401.1; calculated for
1
C25H23NO2S2þ 401.14. H NMR (400 MHz, CDCl3/TFA) d 8.84 (d,
6.1 Hz, 1H), 8.65-8.62 (m, 3H), 8.15-8.10 (m, 2H), 8.00 (d, 9.0 Hz, 4H),
7.19 (d, 9 Hz, 4H), 4.35 (s, 3H), 3.98 (s, 6H). 13C NMR (100 MHz,
CDCl3/Trifluoroacetic acid) d 171.2, 167.3, 151.3, 149.7, 148.0,
147.1, 131.2, 130.1, 129.7, 128.1, 126.0, 117.2, 56.4, 48.2.
3-(1-Methylpyridinium-2-yl)-1,5-di-p-chlorophenylpentane-1,5-
dione
Radical cation generation
1,5-Di-p-chlorophenyl-3-(2-pyrydyl)pentane-1,5-dione (736 mg,
1.85 mmol) and trimethyloxonium tetrafluoroborate (315 mg,
2.13 mmol) were stirred in 5 ml of chloroform for 3 h. The solvent
was removed and the solid left behind was suspended in ether
and stirred for ½ h. It was then filtered and the solid obtained was
recrystallized from ethanol to give 705 mg (66%) of colorless
crystals, mp 211–212 8C. IR (KBr Pellet) 3073, 1693, 1592, 1093,
1057, 997, 827, 796, 524 cmꢀ1. 1H NMR (400 MHz, CD3CN) d 8.67
(d, 6.2 Hz, 1H), 8.31 (t, J ¼ 7.9 Hz, 1H),7.99 (d, 8.2 Hz, 1H), 7.90 (d,
8.5 Hz, 4H), 7.76 (t, 7.0 Hz, 1H), 7.51 (d, 8.5 Hz, 4H), 4.75 (s, 3H), 4.36
(tt, J ¼ 7.6 Hz, 6.5 Hz, 1H), 3.85-3.74 (m, 4H). 13C NMR (100 MHz,
CD3CN) d 198.1, 163.5, 147.2, 146.6, 140.6, 135.5, 130.8, 130.0,
127.5, 126.3, 45.1, 45.2, 33.0.
The radical cations of thiopyrylogens 4a-c2R were produced by
treating 3 ml solutions of the corresponding thiopyrylogen in
acetonitrile (2.72 ꢃ 10ꢀ5 M) with 27mg of zinc dust in an inert
atmosphere. Thiswasaccomplishedbyusingahomemadecellthat
allowedfreeze–pump–thawcyclesofthethiopyrylogeninoneside
arm followed by tipping to mix with the zinc dust in a quartz side
arm of the cell. The characteristic color of the radical cation formed
immediately and continued to develop as monitored by UV–visible
spectroscopy for approximately 1 h 45 min. The radical cation color
disappeared rapidly when the cell was opened to air. The radical
cations for ESR studies were prepared by mixing 0.3 mg of
thiopyrylogen, 20 mg of zinc dust, ꢄ0.25 ml of acetonitrile, and
ꢄ1 ml of toluene in an ESR tube inside a glove box.
N-Methyl-2,6-di-p-chlorophenyl-4,20-thiopyrylogen
bis-tetrafluoroborate 4b2R
Laser flash photolysis
3-(1-Methylpyridinium-2-yl)-1,5-di-p-chlorophenylpentane-1,5-
dione (356 mg, 0.71 mmol) and triphenylmethanol (240 mg,
0.92 mmol) were added to acetic anhydride (2 ml) and H2S(g)
was bubbled through this mixture while heating to 70 8C and
stirring for 7 h. The reaction mixture was then cooled to room
temperature and HBF4 (48% in water, 600 ml) was added slowly all
while maintaining a steady flow of H2S(g). This mixture was
stirred at room temperature for 45 min, then heated to 70 8C, and
stirred for an additional 3.5 h. The supply of H2S(g) was then
turned off and heating was continued for an additional 1 h.
Anhydrous ether was added to the reaction mixture at room
temperature to induce the crystallization of the crude yellow
product. The product was then recrystallized from a mixture of
acetonitrile/acetic acid to give 230 mg of yellow crystalline
material (56%). mp 249–250 8C (decomposes). IR (KBr Pellet) 3092,
1588, 1417, 1266, 1060 cmꢀ1. ESI-MS found 409.3; calculated for
Laser flash photolysis samples were prepared by adjusting the
optical density at 355 nm to 0.3–0.6. The samples were then
degassed by a series of three freeze–pump–thaw cycles in a
custom designed cell with a pyrex side-arm for degassing and a
quartz side-arm for data collection. The samples were irradiated
with the 355 nm output of a Nd:YAG with pulse energies of 7 mJ.
Transient decay curves were generally fitted to first-order kinetics
to get the lifetime and transient absorption spectra were
recorded from 300–700 nm.
Phosphorescence measurements
Phosphorescence spectra and lifetimes of 10ꢀ4 M 4a–c2R were
measured at 77 K in a HCl(g) saturated ethanol glass. All the
pyrylogens were irradiated at their absorbance lMAX using an
excitation slit width of 5 nm and emission slit width of 10 nm.
J. Phys. Org. Chem. 2011, 24 22–28
Copyright ß 2010 John Wiley & Sons, Ltd.
View this article online at wileyonlinelibrary.com

A. K. S. WARRIER AND E. L. CLENNAN
Cyclic voltametry (CV)
Acknowledgements
An acetonitrile solution containing the corresponding pyrylogen
(0.002 M), internal reference (Ferrocene, 0.002 M), and the
supporting electrolyte (tetrabutylammonium perchlorate,
0.1 M) was used for the CV experiments. A three-electrode
experimental design consisting of a glassy carbon working
electrode, a silver/silver-nitrate/acetonitrile reference electrode,
and a platinum counter electrode was used for the CV
measurements. The glassy carbon electrode was frequently
polished in order to remove any deposits on the electrode
surface and the experiments were run in both an inert argon and
oxygen atmosphere. The experiments were repeated for various
The authors thank the National Science Foundation for their
generous support of this research.
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TheESI-Massanalysis wasperformedusinga Thermo-FinniganLCQ
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mMsolutionsofthecompoundsdissolvedinacetonitrilecontaining
0.01% trifluoroacetic acid by direct infusion at the flow rate of
3 ml minꢀ1 in the positive ion mode. LCQ conditions: nitrogen
auxiliary gas 100 au, helium sheath gas 100 au, ion spray voltage
4.5 kV, capillary voltage 10V, capillarytemperature2008C, andtube
lens offset ꢀ10V. The base peak in all cases corresponds to the one
electron reduced radical cation. A similar phenomenon has also
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NMR data, computational details, ESI-MS data, and ESR of 4aR..
These materials are available at the epoc website in Wiley
Interscience.
View this article online at wileyonlinelibrary.com
Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 22–28