Syntheses, characterizations, and properties of electronically perturbed 1,1′-dimethyl-2,2′-bipyridinium tetrafluoroborates

Article abstract of DOI:10.1021/jo052127i

The syntheses of three new 2,2′-bipyridinium tetrafluoroborate sensitizers are reported. Their preliminary electrochemical and photophysical properties are compared to the properties of the more widely used pyrylium cation sensitizers. In addition, the first examples of triplet-triplet absorption spectra of 2,2′-bipyridinium ions are presented.

Full text of DOI:10.1021/jo052127i

Syntheses, Characterizations, and Properties of Electronically
Perturbed 1,1′-Dimethyl-2,2′-bipyridinium Tetrafluoroborates
Dong Zhang, Eric J. Dufek, and Edward L. Clennan*
Department of Chemistry, UniVersity of Wyoming, 1000 East UniVersity AVenue, Laramie, Wyoming 82071
clennane@uwyo.edu
ReceiVed October 11, 2005
The syntheses of three new 2,2′-bipyridinium tetrafluoroborate sensitizers are reported. Their preliminary
electrochemical and photophysical properties are compared to the properties of the more widely used
pyrylium cation sensitizers. In addition, the first examples of triplet-triplet absorption spectra of 2,2′-
bipyridinium ions are presented.
SCHEME 1. Monocationic and Dicationic Sensitizers
Introduction
Photoinduced electron transfer (PET) is a convenient and
well-established methodology.1 Back electron transfer (BET)
in the ion pair intermediates, however, provides a significant
limitation in terms of achieving energy efficiency. This problem
has been addressed by using pyrylium salts such as TPPx to
produce substrate radical cations that do not suffer detrimental
Coulombic attraction to the reduced sensitizer2 (eq 1; Scheme
1).
We recently became interested in determining if BET could
be further suppressed by using dicationic sensitizers such as
viologens3 and their 2,2′-bipyridinium isomers (e.g., MV2+ and
MQ2+ in Scheme 3) to generate the substrate radical cation in
a Coulombically repulsive environment. (eq 2). However,
despite the fact that viologens have been actively explored as
herbicides,3 as components of electrochromic display devices,4
as acceptors in molecular assemblies designed to store solar
energy, as components in supramolecular structures,4 and as
probes to study DNA5 and zeolites6 their use as photochemical
sensitizers has rarely been explored. This is due in large part to
both the absence of photophysical studies7 and the lack of readily
available viologens and 2,2′-bipyridinium ions with convenient
optical and photophysical properties. In addition, only a handful
of reports8-12 deal with bipyridinium ions with nuclear substi-
tutents (on carbon) other than simple alkyl groups or alkyl chains
acting as tethers to supramolecular structures, and as a conse-
quence a series of viologens with a range of oxidizing abilities
is not available.
Results and Discussion
We report here the synthesis of three new first-generation
electronically perturbed 2,2′-bipyridinium ions, 1, 2, and 3
(Scheme 2), and preliminary studies on their photophysical and
oxidizing properties designed to evaluate their viabilities as
electron-transfer sensitizers. We also compare these 2,2′-
bipyridinium ions to the more firmly established electron-
transfer sensitizer TPPx that has several attractive features that
(1) Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.;
Elsevier Science Publishers B. V.: Amsterdam, The Netherlands, 1988.
(2) Miranda, M. A. Chem. ReV. 1994, 94, 1063-1069.
(3) Summers, L. A. The Bipyridinium Herbicides; Academic Press: New
York, 1980.
(4) Monk, P. M. S. The Viologens. Physicochemical Properties, Synthesis
and Applications of the Salts of 4,4′-Bipyridine; John Wiley & Sons:
Chichester, England, 1998.
(5) Brun, A. M.; Harriman, A. J. Am. Chem. Soc. 1991, 113, 8153-
8159.
(6) Clennan, E. L. Coord. Chem. ReV. 2004, 248, 477-492.
(7) For a rare example, see: Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C.;
Luk, Y. F.; Kohler, B. J. Phys. Chem. A 2001, 105, 5768-5777.
(8) Aziz, D.; Breckenridge, J. G. Can. J. Res. 1950, 28B, 26-33.
(9) Downes, J. E. J. Chem. Soc. (C) 1967, 2192-2193.
(10) Downes, J. E. J. Chem. Soc. (C) 1967, 1491-1493.
(11) Fielden, R.; Summers, L. A. Experientia 1974, 30, 843-844.
(12) Pirzada, N. H.; Pojer, P. M.; Summers, L. A. Z. Naturforsch. 1976,
31B, 115-121.
10.1021/jo052127i CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/17/2005
J. Org. Chem. 2006, 71, 315-319
315

Zhang et al.
TABLE 1. Comparison of DFT and X-ray Structures of 1a
SCHEME 2. Syntheses of Bipyridinium Ions 1, 2, and 3a
Erel
kcal/mol
,
d(C6-C7),
θ(C5C6C7C8),
Å
deg
X-ray
1.494(3)
1.501
1.502
81.9(3)
88.06
85.44
86.62
1(out-out)b
1(in-out)
1(in-in)
0
2.0
4.1
a H2O2, CH3COOH, 75 °C. bFuming H2SO4-HNO3, 100 °C, 8 h.
cNaOCH3, CH3OH, 30-35 °C, 5 h. dPCl3, CHCl3, reflux, 5 h. eCH3COCl,
CH3COOH, 100 °C, 4 h. fPCl3, CHCl3, reflux, 6 h. g(CH3)3O+BF4-, CH2Cl2,
1.503
h
i
RT, 36 h. CrO3, H2SO4, 75 °C, 4 h, then RT for 12 h. H2SO4, CH3OH.
a DFT calculations at the B3LYP/6-31G(d) level. b Out and in refer to
the C13O1C4C5 or C14O2C9C8 dihedral angle of near 180° and 0°,
respectively.
have enhanced its popularity as an electron-transfer sensitizer.
These include the following: (1) two absorptions in CH2Cl2 at
417 (ꢀ ) 29 000 L mol-1 cm-1) and 369 nm (42 500 L mol-1
cm-1); (2) singlet and triplet energies of 65 and 53 kcal/mol,
respectively; (3) its inability to produce 1O2 (1∆g) or superoxide;
(4) a large intersystem crossing (S1 f T1) quantum yield of
0.48; and (5) reversible CV reductions in CH3CN at -0.29 and
-1.42 V vs SCE.13
SCHEME 3. Viologen and 2,2′-Bipyridinium Ion Redox
States
The critical step in the syntheses of bipyridinium ions 1, 2,
and 3 (Scheme 2) is the final alkylation with trimethyloxonium
tetrafluoroborate (Meerwein’s salt).14 Early alkylation attempts
with methyl iodide met with failure. In the case of the bipyridine
precursor to 1 several products presumably from adventitious
dealkylation of the methoxy groups were observed.15 The
diminished nucleophilicity of the bipyridine precursors of 2 and
3 precluded reaction, at atmospheric pressure, with methyl iodide
even at extended reaction times. The reaction of the bipyridine
precursors with Meerwin’s salt, however, proceeded to give the
bipyridinium ions in excellent yields (1, 88%; 2, 79%; 3, 84%)
after recrystallization from CH3CN/Et2O or CH3CN/ethyl acetate
mixtures.
Perhaps the single most important feature of viologens and
2,2′-bipyridinium ions is their ability to exist in three well-
defined oxidation levels. These redox states have been most
exhaustively explored with the 4,4′-bipyridinium ion, methyl
viologen MV2+, which exhibits two reversible reductions at
-0.45 and -0.86 V versus SCE.4 Structures for similar
oxidation states for the 2,2′-bipyridinium ions 1, 2, and 3 can
also be drawn (Scheme 3). However, preliminary electrochemi-
cal studies (Figure 1) illustrate that only 3 shows two reversible
one-electron reductions reminiscent of the 4,4′-bipyridinium
ions. All the CV’s were collected under an argon atmosphere
An X-ray crystal structure of 1 is compared to B3LYP DFT
calculations in Table 1. Three methoxy rotomeric minima were
located on the DFT potential energy surface. The X-ray structure
corresponds to the in-out methoxy conformation which is 2.0
kcal/mol above the DFT out-out global minimum (Table 1).
The DFT calculations predict, and the X-ray structure confirms,
a nearly perpendicular alignment of the two pyridinium rings.
-
although only 1+• is thermodynamically competent to reduce
The two BF4 counterions exhibit weak interactions with the
Red
oxygen (E (oxygen) ) -0.86 V vs SCE in CH3CN).16 We
bipyridinium nucleus. Both interactions involve a bridging
fluorine atom: in one case with a methoxy group (O2-F2 2.778
Å) and in the other case with two nitrogen atoms of adjacent
viologen rings (N2-F5 3.211 Å and N1A-F5 3.128 Å) as
denoted by the dotted lines in the structure in Table 1.
1/2
attribute the apparent instability of MQ+. (Scheme 3) to steric
effects that prevent formation of the electronically preferred
planar radical cation. B3LYP/6-31G(d) DFT studies also show
that 3+• is nonplanar; however, the carbomethoxy groups must
provide sufficient resonance interaction to produce a radical
cation stable on the cyclic voltammetry time scale. Bipyridinium
(13) Saeva, F. D.; Olin, G. R. J. Am. Chem. Soc. 1980, 102, 299-303.
(14) Meerwein, H. In Organic Syntheses.; Baumgarten, H. E., Ed.; John
Wiley & Sons: New York, 1973; Collect. Vol. V, pp 1096-1098.
(15) Pojer, P. M.; Summers, L. A. J. Heterocycl. Chem. 1974, 11, 303-
305.
(16) Sawyer, D. T. Oxygen Chemistry; Oxford University Press: New
York, 1991.
316 J. Org. Chem., Vol. 71, No. 1, 2006

1,1′-Dimethyl-2,2′-bipyridinium Tetrafluoroborates
the fluorescence origin of this emission. This confirmation is
important in view of the controversial emission from MV2+ that
has been attributed, by some authors, to adventitious pyridone
oxidation products.19 Only recently has a femtosecond laser flash
photolysis study provided convincing evidence for fluorescence
1
from MV2+* in acetonitrile.7
The quantum yields of fluorescence, ΦF (Table 2), were
determined by comparison with anthracene in ethanol (ΦF )
0.27 ( 0.03) and are considerably smaller than the values of
0.04 ( 0.02 reported for 1,1′-ethylene-2,2′-bipyridinium dibro-
mide in water20 or of 0.03 ( 0.01 reported for MV2+ in
acetonitrile.8-12 We estimate that the S1 energies for both 1 and
2 are approximately 99 kcal/mol from intersection of the
corrected emission and absorption spectra at 290 nm. The singlet
energies are considerably larger than that observed for TPPx
(vide supra) and slightly larger than that reported for MV2+
(95 kcal/mol).8-12 A direct consequence of these high excited
singlet state energies is that these viologens function as more
potent photochemical oxidants than 1TPPx*. For example, since
FIGURE 1. Cyclic voltammograms for 1, 2, 3, and MQ2+
.
TABLE 2. Optical Data for 2,2′-Bipyridinium Ions 1, 2, and 3
∆G° (kcal mol-1) for electron transfer is given by 23.06(EOx
-
c
e
λ
ABS (ꢀ)a
λfb
ΦF
λP
(donor) - ERed(acceptor) - ∆ESinglet), 1TPPx* is thermodynami-
cally capable of oxidizing substrates (donors) with oxidation
potentials less than 2.5 V vs SCE while 11* can oxidize
substrates with EOx as high as 3.32 V vs SCE.
1
2
212 (40 800)
251 (33 200)
200 (41 200)
241 (24 100)
263 (11 900)
271 (13 800)
224 (26,200)
285 (17 900)
345
0.0010 ( 0.0004
392
332
0.012 ( 0.002
425
Figure 2 shows transient absorption spectra collected 0.56
and 0.28 µs after irradiation of 0.1 mM solutions of 2 and 3,
respectively, with 266 nm pump pulses from a Nd:YAG laser.
The absorption spectra exhibit valleys that we attribute to
bleaching of 2 and 3 and absorption bands at approximately
430 and 450 nm, respectively. In samples rigorously degassed
by a series of freeze-thaw cycles at 10-4 Torr these signals
decay by first-order processes (insets in Figure 2) with lifetimes
of 22 ( 1 (2) and 4.8 ( 0.5 µs (3). The signals do not decay
completely to the baseline and small residual signals (see insets
in Figure 2) exist out to the millsecond time regime.
Photoreduction of MV2+ has been reported in zeolites, in
methanol,20,21 and as the chloride salt under conditions which
favor tight ion pairing. However, we can rule out the reduction
products, 2+• and 3+•, as the species responsible for the transient
absorption spectra (Scheme 5) for several reasons: (1) The
observed lifetimes are inconsistent with the cyclic voltammetry
results which require substantially longer lifetimes for the radical
cations, (2) RPA/6-31G(d) calculations predict that MQ+• should
absorb at 985 nm.18 It is unlikely that substituent effects could
be responsible for over a 500 nm hypsochromic shift, and (3)
despite the fact that 2+• and 3+• are thermodynamically
precluded from producing superoxide, oxygen dramatically
enhances the rate of decay of the transients. Consequently, as
an alternative we assign the observed transients as triplet-triplet
absorption spectra. This assignment is confirmed by the
observation that 1 is capable of sensitizing the singlet oxygen
ene reaction of 2,3-dimethyl-2-butene to give the allylic
hydroperoxide product.22,23 It is also consistent with the oxygen
quenching results and the observation of phosphorescence at
77 K (Table 1).
3
d
432
a Absorption maxima, λABS, in nm; extinction coefficient, ꢀ, in L mol-1
cm-1 in CH3CN. b Fluorescence maximum in CH3CN. c Fluorescence
quantum yield in CH3CN versus anthracene in ethanol (ΦF ) 0.27(0.03).
d Not observed. e Phosphorescence maximum in ethanol glass at 77 K.
ion 1 gives a single 2-electron reduction, as determined
coulometrically, at -0.97 V versus SCE, while 2 exhibits two
CV peaks with only a hint of reversibility.
Despite this wide spectrum of electrochemical behavior we
note that the substituents provide the anticipated electronic effect
on the formation of the radical cations since the reduction
potentials given in Figure 1 are linearly related to the Hammett
substituent constants (Ered(1) ) 0.95σp - 0.745; r2 ) 0.9631).
Optical data collected for bipyridinium ions 1, 2, and 3 are
given in Table 2. Unfortunately, all of the bipyridinium ions
absorb at considerably shorter wavelengths than TPPx. Addition
of 2,3-dimethyl-2-butene to the 2,2′-bipyridinium ions resulted
in formation of a shoulder on the red edge of each absorption
spectrum consistent with charge transfer (CT) formation.
Benesi-Hildebrand analysis17 of the charge-transfer complexes
in CH3CN gave equilbrium constants (1, 0.16 ( 0.06 M-1; 2,
0.58 ( 0.08 M-1; 3, 0.20 ( 0.03 M-1) of the same order of
magnitude as observed previously for MQ2+ 18
The lack of
.
correlation of Keq with the electronic character of the substituent
suggests that steric effects also play a role in the stabilities of
these CT complexes.
2,2,-Bipyridinium ions 1 and 2, but not 3, also generate very
weak fluorescence at approximately 345 and 332 nm, respec-
tively. The excellent agreement between the ground-state
absorption and the excitation spectra for both 1 and 2 confirms
(19) Mau, A. W.-H.; Overbeek, J. M.; Loder, J. W.; Sasse, W. H. F. J.
Chem. Soc. Faraday Trans. 2 1986, 82, 869-876.
(20) Hopkins, A. S.; Ledwith, A.; Stam, M. F. J. Chem. Soc., Chem.
Commun. 1970, 494-495.
(21) Ledwith, A. Acc. Chem. Res. 1972, 5, 133-139.
(22) Clennan, E. L. Tetrahedron 2000, 56, 9151-9179.
(23) Clennan, E. L. In Synthetic Organic Photochemistry; Marcel
Kekker: New York, 2005; pp 365-390.
(17) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703-
2707.
(18) Pace, A.; Clennan, E. L.; Jensen, F.; Singleton, J. J. Phys. Chem. B
2004, 108, 4673-4678.
J. Org. Chem, Vol. 71, No. 1, 2006 317

Zhang et al.
solvent was evaporated in vacuo and the product recrystallized three
times from CH3CN and ethyl acetate to give colorless crystal.
Yield: 0.37 g (0.88 mmol, 88%). Mp: 195-196 °C. 1H NMR (400
MHz; D2O, ppm): δ 8.86 (d, J ) 7.3 Hz, 2H), 7.76 (d, J ) 3.0
Hz, 2H), 7.72 (dd, J ) 7.3 and 3.0 Hz, 2H), 4.18 (s, 6H), 3.97 (s,
1
6H). H NMR(400 MHz; CD3CN, ppm): δ 8.69 (d, J ) 6.8 Hz,
2H), 7.63 (m, 4H), 4.17 (s, 6H), 3.89 (s, 6H). 13C NMR(100 MHz;
CD3CN, ppm): δ 173.1 (s), 151.0 (d, JC-H ) 193 Hz), 145.8 (s),
118.3 (d, JC-H ) 175 Hz), 115.9 (d, JC-H ) 176 Hz), 59.9 (q,
JC-H )149), 46.4 (q, JC-H ) 146 Hz). IR: Vmax (KBr pellet) 3090
(w), 1640 (m), 1520 (m), 1280 (m), 1060 (s) cm-1. Anal. Calcd
for C14H18N2O2B2F8: C, 40.04; H, 4.32; N, 6.67. Found: C, 40.30;
H, 4.27; N, 6.61.
4,4′-Dichloro-1,1′-Dimethyl-2,2′-bipyridinium Tetrafluorobo-
rate (2).24 A mixture of 4,4′-dichloro-2,2′-bipyridine (0.001 mol)
and trimethyloxonium tetrafluoroborate (0.002 mol) in 30 mL of
dichloromethane was stirred at room temperature for 48 h. The
solvent was evaporated in vacuo and the product recrystallized three
times from CH3CN and diethyl ether to give a white product.
Yield: 0.34 g (0.79 mmol, 79%). Mp >225 °C. 1H NMR(400 MHz;
CD3CN, ppm): δ 8.94 (d, J ) 6.8 Hz, 2H), 8.38 (dd, J ) 6.7 and
2.5 Hz, 2H), 8.25 (d, J ) 2.3 Hz, 2H), 4.07 (s, 6H). 1H NMR(400
MHz; D2O, ppm): δ 9.19 (d, J ) 6.1 Hz, 2H), 8.49 (m, 4H), 4.19
(s, 6H). 13C NMR(100 MHz; CD3CN, ppm): δ 156.9 (s), 151.4
(d, JC-H ) 198 Hz), 144.0 (s), 133.2 (d, JC-H ) 188 Hz), 132.5 (d,
JC-H ) 182 Hz), 48.8(q, JC-H ) 148 Hz). IR: Vmax (KBr pellet)
3100 (m), 1620 (m), 1060 (s) cm-1. Anal. Calcd for C12H12N2B2F8-
Cl2: C, 33.62; H, 2.82; N, 6.53. Found: C, 33.73; H, 2.77; N,
6.52.
1,1′-Dimethyl-4,4′-dicarbomethoxy-2,2′-bipyridinium Tet-
rafluoroborate (3).24 A mixture of 4,4′-dicarbomethoxy-2,2′-
bipyridine (0.001 mol) and trimethyloxonium tetrafluoroborate
(0.002 mol) in 30 mL of dichloromethane was stirred at room
temperature for 36 h. The solvent was evaporated in vacuo and the
product recrystallized three times from CH3CN and ethyl acetate
to give white crystal. Yield: 0.40 g (0.85 mmol, 84%). Mp: 150
FIGURE 2. Nanosecond laser flash photolysis data for 2 and 3.
Transient absorption spectra and decay profiles (inset) for (a) 2 and
(b) 3 in CH3CN monitored at 450 nm for 2 and 460 nm for 3.
1
°C dec. H NMR (400 MHz; CD3CN, ppm): δ 9.20 (d, J ) 6.3
Hz, 2H), 8.73 (dd, J ) 6.3 and 2.0 Hz, 2H), 8.66 (d, J ) 1.9 Hz,
2H), 4.15 (s, 6H), 4.05 (s, 6H). 13C NMR(100 MHz; CD3CN,
ppm): δ 162.9 (s), 152.5 (d, JC-H ) 201 Hz), 147.6 (s), 144.7 (s),
131.8 (d, JC-H ) 174 Hz), 131.4 (d, JC-H ) 179 Hz), 55.3 (q,
JC-H )149 Hz), 49.6 (q, JC-H ) 148 Hz). IR: Vmax (KBr pellet)
1750 (s), 1300 (m), 1070 (s), 768 (m), 525 (s) cm-1. Anal. Calcd
for C16H18N2O4B2F8: C, 40.38; H, 3.81; N, 5.89. Found: C, 40.08;
H, 3.76; N, 5.74.
We estimate from the phosphorescence spectra that the triplet
energies are approximately 75-80 kcal/mol. Consequently, the
triplets should be capable of oxidizing substrates with oxidation
potentials less than 3.0 V vs SCE.
Conclusion
We have reported the synthesis of three new first-generation
2,2′-bipyridinium salts with electronically pertubing substituents
directly attached to the carbons in the pyridinium core. An
optical study of these new compounds has resulted in the first
example of triplet-triplet absorption spectra of 2,2′-bipyridinium
ions and electrochemical properties that span a convenient
potential range. However, their absorption maxima are substan-
tially hypsochromic of the more widely used pyrylium salts
decreasing their utility as electron transfer sensitizers. Conse-
quently, syntheses and characterizations of new second-genera-
tion electron transfer sensitizers bathochromatically shifted from
typical substrate absorbances are currently underway and will
be reported soon.
Fluorescence Quantum Yield Determinations. The quantum
yields were determined by the method of Eaton.25 The quantum
yield of the fluorescence standard, anthracene, in ethanol (ΦA
)
0.27 ( 0.03)10,25 was used in conjunction with the equation ΦF )
2
[(AAFVn2)/(AVFAn0 )]ΦA to determine the quantum yield of 2,2′-
bipyridinium ion fluorescence, ΦF, where AA and AV are the
absorbance at the excitation wavelength, FA and FV are the
integrated emission area across the band, of anthracene and the
viologen, respectively, and n0 and n are the index of refraction of
ethanol (1.3600) and acetonitrile (1.3410), respectively. The ab-
sorbances of the 2,2′-bipyridinium ions and anthracene were
adjusted to be similar and below 0.10. The reported results are the
average of three independent determinations. Anthracene in etha-
nol: slit width ) 2.5 nm, excitation wavelength ) 345 nm, A345nm
) 0.09546, F ) 8020. 2,2′-Bipyridinium ion, 1, in acetonitrile:
slit width ) 2.5 nm, excitation wavelength ) 253 nm, [1] ) 3.12
× 10-6 M, A253 nm ) 0.1000, F ) 32. 2,2′-Bipyridium ion, 2, in
acetonitrile: slit width ) 2.5 nm, excitation wavelength ) 272 nm,
[2] ) 3.90 × 10-6 M, A272 nm ) 0.0553, F ) 209.
Experimental Section
4,4′-Dimethoxy-1,1′-dimethyl-2,2′-bipyridinium Tetrafluo-
roborate (1).24 A mixture of 4,4′-dimethoxy-2,2′-bipyridine (0.001
mol) and trimethyloxonium tetrafluoroborate (0.002 mol) in 30 mL
of dichloromethane was stirred at room temperature for 36 h. The
X-ray Structure of 1. X-ray diffraction data from a crystal of
1 obtained by slow crystallization from 1:1 CH3CN:ethyl acetate
(24) Negele, S.; Wieser, K.; Severin, T. J. Org. Chem. 1998, 63, 1138-
1143.
(25) Eaton, D. F. In CRC Handbook of Organic Photochemistry; Scaiano,
J. C., Ed.; CRC Press: Boca Raton, FL, 2000; Vol. I, pp 231-239.
318 J. Org. Chem., Vol. 71, No. 1, 2006

1,1′-Dimethyl-2,2′-bipyridinium Tetrafluoroborates
were collected on a diffractometer employing Mo KR radiation
(graphite monochromator) at -100 °C. Standard control and
integration and structure solution, refinement, and graphics software
were employed. The unit cell parameters were obtained from a least-
squares fit to the angular coordinates of all reflections. Intensities
were integrated from a series of frames (0.3° ω rotation) covering
more than a hemisphere of reciprocal space. Absorption and other
corrections were applied.
were collected with use of slit widths of 5 nm and with PMT
voltages set at 775 V. Samples were prepared and the data collected
in absolute ethanol that had been purified by atmospheric distillation
through a 1 m column packed with glass beads. The sample
concentrations were 2 × 10-6, 2 × 10-6, and 8 × 10-6 M for 1, 2,
and 3, respectively. Phosphorescence data were collected by
excitation at 2540, 2720, and 2900 Å for samples 1, 2, and 3,
respectively. The phosphorescence spectra were generated from
these data by subtracting background emission data from ethanol
blanks excited at identical wavelengths.
1 crystallizes in the orthorhombic space group, Pbca. The
-
asymmetric unit consists of the bipyridinium cations and two BF4
Cyclic Voltammetry. Data were collected at 1000 mV/s (250
mV/s for 3) in acetonitrile with samples 2 × 10-3 M in substrate
and 0.1 M in tetra-n-butylammonium perchlorate on a Potentiostat/
Galvanostat. The working electrode was glassy carbon and the
counter electrode was a platinum wire. The reference electrode was
Ag/AgCl 0.1 M KCl in water, which were converted to SCE by
subtraction of 0.047 V.26 Preliminary work at different scan rates
has also been done. Expected changes as anticipated for irreversible
and quasireversible systems were observed.
anions. All of them are well-ordered and situated on general
positions. All non-hydrogen atoms were located in the Fourier maps
and refined anisotropically. All H atoms were placed in calculated
positions and refined isotropically by a riding model.
Phosphorescence Spectra. Low-temperature phosphorescence
spectra were obtained with a resolution of (1 nm at 77 K, using
the pulsed source and gated detection in a spectrometer equipped
with a low-temperature accessory. The delay times were set at 0.1
ms and gate times at 9.9 ms for each sample. The emission data
DTF Calculations. DFT calculations were done with Gaussian
0327 and the characters of all stationary points were verified by
frequency calculations.
(26) Handbook of Analytical Chemistry; McGraw Hill: New York, 1963;
Section 5.
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Honda,
H.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.;
Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D. ; Raghavachari, K.; Foresman, J. B.; Ortiz,
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J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
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Pople, Gaussian 03, Revision A.1;Gaussian, Inc., Pittsburgh, PA, 2003.
Acknowledgment. We thank the National Science Founda-
tion (CHE-0313657) for the generous support of this research.
We also thank Sean Hightower for assistance in the DFT
calculations.
Supporting Information Available: Synthetic procedures,
several ORTEP views of crystal and unit cell, and Z-matrix and
total energies for calculations. as well as a CIF file for the crystal
structure of 1. This material is available free of charge via the
Internet at http://pubs.acs.org.
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