Photoinduced electron transfer in covalently linked 1,8-naphthalimide/viologen systems

Article abstract of DOI:10.1021/jp000855y

A series of polymethylene-linked 1,8-naphthalimide/viologen diads has been synthesized. The number of intervening methylenes was varied from 2 to 6. For comparison, a series of W-alkylpyridiniumyl-1,8-naphthalimide "parent" compounds was prepared and phot

Full text of DOI:10.1021/jp000855y

6778
J. Phys. Chem. A 2000, 104, 6778-6785
Photoinduced Electron Transfer in Covalently Linked 1,8-Naphthalimide/Viologen Systems
Thao P. Le, Joy E. Rogers, and Lisa A. Kelly*
Department of Chemistry and Biochemistry, UniVersity of Maryland, Baltimore County, 1000 Hilltop Circle,
Baltimore, Maryland 21250
ReceiVed: March 3, 2000; In Final Form: May 18, 2000
A series of polymethylene-linked 1,8-naphthalimide/viologen diads has been synthesized. The number of
intervening methylenes was varied from 2 to 6. For comparison, a series of N-alkylpyridiniumyl-1,8-
naphthalimide “parent” compounds was prepared and photophysically characterized. Relative to the parent
compounds, the electronically excited singlet state of the 1,8-naphthalimide was found to be quenched by the
covalently attached viologen. From Stern-Volmer analyses of the steady-state fluorescence spectra, along
with the singlet-state lifetime of the pyridinium-substituted 1,8-naphthalimide, the rate constants for
intramolecular quenching were calculated to range from 1.5 × 1010 s-1 (2 intervening methylenes) to 8.3 ×
107 s-1 (6 intervening methylenes) in aqueous buffered solution. For comparison, the intermolecular reactivity
of the excited singlet state of N-alkylpyridiniumyl-1,8-naphthalimides with methylviologen was assessed. In
0.5 M phosphate buffer (pH 7.0), the bimolecular rate constant was found to be 3.2 × 109 M-1 s-1. Nanosecond
laser flash photolysis studies were carried out to identify the quenching products. From these studies, reduced
methylviologen was identified as a singlet-state quenching product. From these results, we attribute both the
intra- and intermolecular quenching process to electron transfer from the singlet excited state of 1,8-
naphthalimide to methylviologen. Within the covalently linked series, the rate constant for intramolecular
electron transfer was found to vary exponentially with the number of intervening methylenes. Linear least-
squares analysis of the results yielded an apparent â value of 1.04 Å-1 for electron transfer through the
polymethylene bridge.
Introduction
extended conformation.7-10 In a series of naphthoxyl-(CH2)n-
viologen donor-acceptor diads, exponential distance depend-
ences were observed at shorter distances (n e 6).7 For the longer
linkages, the nonexponential behavior was corrected by com-
plexation with cyclodextrin. In a separate study, rate constants
for electron transfer from the MLCT excited state of a tris(2,2′-
bipyridyl)ruthenium(II) complex to a covalently attached vi-
ologen were measured as a function of the number of intervening
methylenes.8 In aqueous solution, exponential distance depen-
dence up to six methylenes was observed. The nonlinearities at
longer separation distances were corrected by encapsulating the
diad in â-cyclodextrin. Although conformational flexibility is
certainly a complication in extracting distance dependencies
through polymethylene chains, these reports suggest that pho-
tophysical perturbations, due to through-space interactions, occur
primarily at longer separation distances.
Photoinduced electron transfer plays a key role in light-driven
chemical, physical, and biological processes. Photosynthesis
represents a noteworthy and ubiquitous model that has motivated
the design of many elaborate assemblies to convert light energy
into chemical potential. To mimic the charge transfer processes
that accompany photosynthesis, covalently linked arrays of
electron donors and acceptors are often used.1,2 From these
synthetic assemblies, the effects of thermodynamic driving force,
distance and intervening medium on electron transfer (ET)
kinetics have been investigated. To best understand the distance
dependence, electron donor-acceptor systems that are linked
by rigid spacing groups have been investigated. Specifically,
the rate constant (kET) for ET through steroidal,3 norbornyl,4 or
peptide5 spacers has shown to decrease exponentially with the
distance (rDA) between the donor and acceptor group according
to kET ) A exp(-â(rDA - r0DA)). In this treatment, â depends
on the nature of the linkage or intervening medium and r0DA is
the distance of closest approach.
Donor-acceptor systems linked by flexible spacing groups,
such as polymethylene chains (-(CH2)n-), are generally easier
to synthesize. However, extracting linkage-dependent â values
from these systems is complicated by the fact that electron
transfer may occur from all conformers in solution.6 To
overcome this difficulty, cyclodextrins capable of forming
inclusion complexes with flexible, covalently linked donor-
acceptor systems have been used to force the assembly into an
In this paper, we characterize the intramolecular electron
transfer kinetics in a series of donor-acceptor diads in which
a 1,8-naphthalimide chromophore is covalently attached to a
viologen electron acceptor. The electron donor and acceptor are
separated by 2-6 intervening methylenes. From the kinetic data,
polymethylene distance dependencies are extracted. For com-
parison, intermolecular electron transfer from the naphthalimide
singlet excited state to methylviologen was also investigated.
The viologens are ubiquitous electron acceptors and have been
coupled to tris(bipyridyl)ruthenium(II) chromophores8,11 and
porphyrins,12-15 to study intramolecular electron transfer.
Likewise, the facile and reversible one-electron reduction of
N-substituted 1,8-naphthalimides has been studied electrochemi-
* Corresponding author: E-mail: lkelly@umbc.edu.
10.1021/jp000855y CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/24/2000

Photoinduced ET in 1,8-Naphthalimide/Viologen Systems
J. Phys. Chem. A, Vol. 104, No. 29, 2000 6779
filtered and dried (yield ) 81%). The pyridinium-substituted
imide was prepared by dissolving C2OH (1.5 g) and 1.4 g of
p-toluenesulfonyl chloride (1.4 g) in 4 mL of dry pyridine and
stirring at room temperature for 12 h. The precipitate that was
formed was collected by vacuum filtration, dissolved in a
minimal amount of water and stirred with DOWEX 1 × 8 200
ion-exchange resin for ca. 30 min to obtain the chloride salt of
C2py. The DOWEX resin was filtered from solution and the
water removed from the filtrate. The resulting product was
recrystallized twice in methanol under an atmosphere of diethyl
ether. The product was obtained in 60% yield after recrystal-
lization. Analytically pure material was obtained by further
purification on a neutral alumina column. Trace impurities were
removed by elution with methanol. The product was then eluted
from the column using 50:50 H2O:methanol. 1H NMR (DMSO-
d6): 9.20 (d, 2H, pyr), 8.60 (t, 1H, pyr), 8.50 (d, 2H, naphth),
8.41 (d, 2H, naphth), 8.09 (t, 2H, pyr), 7.86 (t, 2H, naphth),
4.96 (t, 2H, CH2), 4.61 (t, 2H, CH2). Anal. Calcd (C19H15N2O2-
Cl): C, 67.36; H, 4.46; N, 8.27. Found: C, 67.02; H, 4.48; N,
8.04.
Figure 1. Synthesis of pyridinium-substituted derivatives of 1,8-
naphthalimide (C2py, C3py, C4py, C5py, and C6py) and donor-
acceptor diad systems (C2vio, C3vio, C4vio, C5vio, and C6vio) used
in this study.
cally.16 Following photon absorption, the electronically excited
singlet and triplet states of these aromatic imides have been
used to oxidize DNA nucleotides and duplex DNA.17,18 In a
separate report, intramolecular electron transfer to the excited
singlet and triplet states of N-substituted 1,8-naphthalimide from
aniline or methoxyaniline has been demonstrated.19 Thus, both
constituents of the naphthalimide-viologen diads are generally
employed as electron acceptors. In this work, we demonstrate
photoinitiated electron transfer from the singlet excited state of
1,8-naphthalimide to the covalently attached viologen and
demonstrate the versatile reactivity of naphthalimide excited
states as reducing agents in photoinduced electron transfer.
N-(3-(N-Pyridiniumyl)propyl)-1,8-naphthalenimide (C3py).
The synthetic and purification procedures were identical to those
used for C2py. The material was obtained in 45% overall yield
1
after recrystallization from methanol. H NMR (DMSO-d6):
9.11 (d, 2H, pyr), 8.56 (t, 1H, pyr), 8.40 (d+d, 4H, naphth),
8.11 (t, 2H, pyr), 7.86 (t, 2H, naphth), 4.68 (t, 2H, CH2), 4.12
(t, 2H, CH2), 2.30 (m, 2H, CH2). Anal. Calcd (C20H17N2O2Cl‚
2H2O): C, 61.78; H, 4.41; N, 7.20. Found: C, 61.85; H, 4.01;
N, 7.20.
N-(4-(N-Pyridiniumyl)butyl)-1,8-naphthalenimide (C4py). The
N-butanol intermediate (N-(4-butanol)-1,8-naphthalenimide
(C4OH)) was prepared in 91% yield using the procedure
described above for C2OH. The dried product was stirred under
nitrogen at room temperature for 12 h with 1.2 equiv of
p-toluenesulfonyl chloride in a minimal amount of dry pyridine.
C4py was precipitated from the pyridine by the addition of THF
and converted to the chloride salt as described above. The
material was dried and purified on a neutral alumina column.
After elution of the impurities with methanol, the desired product
was eluted from the column using 30:70 H2O:methanol. The
material was recrystallized from methanol in an atmosphere of
Experimental Section
Materials. 1,8-Naphthalic anhydride (97%, Acros) was
recrystallized twice from N,N-dimethylacetamide (DMA) and
dried in vacuo prior to use. 2-Amino-1-ethanol (Aldrich, 99+%),
3-amino-1-propanol (Acros, 99%), 4-amino-1-butanol (Acros,
98%), 5-amino-1-propanol (Acros, 97%), and 6-amino-1-
hexanol (Acros, 94%) were used as received. 4,4′-Dipyridyl
(99.8%), p-toluenesulfonyl chloride (99+%), and methyl iodide
(99%) were obtained from Aldrich and used as received.
Tetrabutylammonium perchlorate (TBAP) (99%) was obtained
from Sigma and dried in vacuo prior to use. Anhydrous pyridine
was obtained from Aldrich and used as received. Spectral grade
acetonitrile (Fluka) was used as received for electrochemical
studies. Methylviologen (dichloride hydrate, 98%) was obtained
from Aldrich and recrystallized from ethanol prior to use.
Elemental analyses were performed by Atlantic Microlabs.
The functionalized naphthalimide compounds used in this
study were synthesized as shown in Figure 1. All of the
pyridinium-substituted 1,8-naphthalimides were isolated and
used as their monochloride salts. The viologen-linked diads were
isolated and used as diiodide salts as described below. The
compounds within the series are named according to the number
of methylene substituents (x) between the naphthalene imide
chromophore and the covalently linked pyridinium (Cxpy) or
viologen (Cxvio).
1
diethyl ether (yield ) 40%). H NMR (DMSO-d6): 9.11 (d,
2H, pyr), 8.56 (t, 1H, pyr), 8.47 (dd, 4H, naphth), 8.14 (t, 2H,
pyr), 7.87 (t, 2H, naphth), 4.64 (t, 2H, CH2), 4.09 (t, 2H, CH2),
2.02 (m, 2H, CH2), 1.67 (m, 2H, CH2). Anal. Calcd (C21H19N2O2-
Cl‚2H2O): C, 62.61; H, 4.75; N, 6.95. Found: C, 62.54; H,
4.81; N, 6.93.
N-(5-(N-Pyridiniumyl)pentyl)-1,8-naphthalenimide (C5py).
The synthetic and purification procedures were identical to those
used for C4py. C5py was obtained in 83% yield from the
alcohol precursor. 1H NMR (DMSO-d6): 9.07 (d, 2H, pyr), 8.56
(t, 1H, pyr), 8.45 (dd, 4H, naphth), 8.12 (t, 2H, pyr), 7.84 (t,
2H, naphth), 4.58 (t, 2H, CH2), 4.01 (t, 2H, CH2), 1.95 (m, 2H,
CH2), 1.65 (m, 2H, CH2), 1.33 (m, 2H, CH2). Anal. Calcd
(C22H21N2O2Cl‚H2O): C, 66.24; H, 5.31; N, 7.02. Found: C,
66.08; H, 5.56; N, 6.95.
N-(6-(N-Pyridiniumyl)hexyl)-1,8-naphthalenimide (C6py).
1-Amino-6-hexanol (34 mmol) in 20 mL of DMA was added
dropwise, over a period of 30 min, to a suspension of
1,8-naphthalic anhydride (17 mmol) in 250 mL of DMA. The
mixture was stirred at 100 °C for 6 h. After removal of the
solvent on a rotary evaporator, water (ca. 100 mL) was added
to the residue. The resulting white precipitate (N-(6-hexanol)-
1,8-naphthalenimide (C6OH)) was filtered and dried (yield )
N-(2-(N-Pyridiniumyl)ethyl)-1,8-naphthalenimide (C2py). 1,8-
Naphthalic anhydride (25 mmol) was suspended in 40 mL of
DMA. 1-Amino-2-ethanol (50 mmol) was added dropwise to
the suspension over 30 min. The mixture was stirred at 100 °C
for 3 h. After removal of the solvent on a rotary evaporator,
water (ca. 25 mL) was added to the residue. The resulting white
precipitate (N-(2-ethanol)-1,8-naphthalenimide (C2OH)) was

6780 J. Phys. Chem. A, Vol. 104, No. 29, 2000
Le et al.
72%). The pyridinium-substituted imide was prepared by
dissolving C6OH (1.50 g) and 1.4 g of p-toluenesulfonyl
chloride (1.15 g) in 4 mL of dry pyridine and stirring at room
temperature for 4 h. The product (precipitated as the chloride
salt) was obtained in 72% yield and was purified by recrystal-
lization from methanol/acetonitrile. 1H NMR (DMSO-d6): 9.12
(d, 2H, pyr), 8.59 (t, 1H, pyr), 8.47 (dd, 4H, naphth), 8.15 (t,
2H, pyr), 7.86 (t, 2H, pyr), 4.61 (t,2H, CH2), 4.02 (t, 2H, CH2),
1.95 (m, 2H, CH2), 1.63 (m, 2H, CH2), 1.36 (m, 4H, CH2).
7.91 (t, 2H, naphth), 4.72 (t, 2H, CH2), 4.57 (s, 2H, CH3), 4.09
(t, 2H, CH2), 2.08 (m, 2H, CH2), 1.74 (m, 2H, CH2), 1.43 (m,
2H, CH2). Anal. Calcd (C28H27N3O2I2‚2H2O): C, 45.86; H, 3.71;
N, 5.73. Found: C, 45.70; H, 3.81; N, 5.62.
N-Methyl-N′-hexyl-4,4′-bipyridiniumyl-1,8-naphthalimide
(C6Wio). Alcohol (C6OH, 5.05 mmol) was dissolved in pyridine
(4 mL) at room temperature. p-Toluenesulfonyl chloride (6.06
mmol) was added in the solution, and the mixture was stirred
at room temperature for 3 h. The white precipitate that formed
was filtered and washed with cold water to yield 63% C6OTs.
C6OTs (1.1 mmol) was refluxed with an equimolar amount of
1-methyl-4,4′-bipyridinium in 10 mL of CH3CN overnight. The
red-orange precipitate that was formed was filtered and washed
with minimum amount of cold acetonitrile (yield ) 38%). The
product was recrystallized two times from methanol. 1H NMR
(DMSO-d6): (C6OTs) 8.42 (dd, 4H, naphth), 7.82 (t, 2H,
naphth), 7.73 (d, 2H, ts), 7.42 (d, 2H, ts), 3.96 (m, 4H, CH2),
N-Methyl-N′-ethyl-4,4′-bipyridiniumyl-1,8-naphthalimide
(C2Wio). C2OH (6.1 mmol) was dissolved in 12 mL of dry
pyridine. One equivalent of tosyl chloride was added, and the
mixture was stirred at 0 °C for 2 h. The white powder was
filtered and washed with cold pyridine to yield 40% of the
tolsylated intermediate (C2OTs). C2OTs (2.0 mmol) and
1-methyl-4,4′-bipyridinium (prepared as previously described)20
(1.6 mmol) were refluxed in CH3CN (20 mL) for 15 h. The
iodide salt was isolated as a red-orange precipitate that was
filtered and washed with a minimum amount of cold acetonitrile
to yield 79% C2vio. Analytically pure product was obtained
after successive (five times) recrystallizations from methanol.
1H NMR (DMSO-d6): (C2OTs) 8.43 (dd, 4H, naphth), 7.84
(t, 2H, naphth), 7.45 (d, 2H, ts), 6.83 (d, 2H, ts), 4.32 (m,
4H, CH2), 2.45 (s, 3H, CH3).1H NMR (DMSO-d6): (C2vio)
9.53 (d, 2H, vio), 9.28 (d, 2H, vio), 8.76 (dd, 4H, vio), 8.51 (d,
2H, naphth), 8.39 (d, 2H, naphth), 7.87 (t, 2H, naphth), 5.04 (t,
2H CH2), 4.68 (t, 2H), 4.42 (s, 3H, -CH3). Anal. Calcd
(C25H21N3O2I2‚2H2O): C, 43.44; H, 3.06; N, 6.08. Found: C,
43.80; H, 3.09; N, 6.14.
1
3.28 (s, 3H, CH3), 1.52 (m, 4H, CH2), 1.22 (m, 4H, CH2). H
NMR (DMSO-d6): (C6vio) 9.37 (d, 2H, vio), 9.28 (d, 2H, vio),
8.76 (dd, 4H, vio), 8.47 (dd, 4H, naphth), 7.87 (t, 2H, naphth),
4.69 (t, 2H, CH2), 4.04 (t, 2H, CH2), 4.43 (s, 3H, CH3), 1.97
(m, 2H, CH2), 1.64 (m, 2H, CH2), 1.38 (m, 4H, CH2). Anal.
Calcd (C29H29N3O2I2): C, 48.96; H, 4.11; N, 5.91. Found: C,
49.45; H, 4.13; N, 5.79
General Techniques. Ground-state UV/vis absorption spectra
were measured using a JASCO V-570 double-beam spectro-
photometer. Proton NMR spectra were obtained using either a
GE QE-300 or a Varian Mercury 200 MHz NMR spectrometer.
Fluorescence spectra were measured using a SPEX Fluoromax-2
fluorescence spectrometer.
N-Methyl-N′-propyl-4,4′-bipyridiniumyl-1,8-naphthalimide
(C3Wio). C3OTs was prepared as described above for C2OTs.
The product was isolated by the addition of cold water to the
pyridine solution and vacuum filtration of the water insoluble
product. C3vio was prepared by refluxing C3OTs with 1.2 equiv
of the iodide salt of N-methyl-4,4′-bipyrdinium in a minimal
amount of CH3CN for 12 h. The red-orange precipitate that was
formed was filtered and washed with cold acetonitrile to yield
C3vio. The product was twice recrystallized from methanol
(yield ) 13%). 1H NMR (DMSO-d6): (C3OTs) 8.43 (dd, 4H,
naphth), 7.85 (t, 2H, naphth), 7.67 (d, 2H, ts), 7.35 (d, 2H, ts),
Fluorescence lifetimes were measured using time-correlated
single-photon counting techniques at the National Synchrotron
Light Source. Excitation light from the quartz-windowed,
bending magnet port of beamline U9B was monochromated and
used for pulsed excitation of the sample. The temporal (with
respect to the excitation pulse) and spectral properties of the
emitted light were simultaneously detected using a resistive-
anode, single-photon counting detector. The optical configura-
tion and detection system have been previously described.21 For
these experiments, 344 nm excitation light was employed.
Fluorescence from the sample, integrated from 370 to 500 nm,
was reconvoluted with the measured instrument response
function to obtain the singlet state lifetimes of the systems.
1
4.11 (m, 4H, CH2), 2.34 (s, 3H, CH3), 1.9 (m, 2H, CH2). H
NMR (DMSO-d6): (C3vio) 9.38 (d, 2H, vio), 9.27 (d, 2H, vio),
8.77 (dd, 4H, vio), 8.52 (dd, 4H, naphth), 7.91 (t, 2H, naph),
4.82 (t, 2H, CH2), 4.43 (s, 3H, CH3), 4.21 (t, 2H, CH2), 2.39
(m, 2H, CH2). Anal. Calcd (C26H23N3O2I2): C, 46.66; H, 3.46;
N, 6.28. Found: C, 46.97; H, 3.44; N, 6.23.
One-electron reduction potentials of the naphthalene imide
derivatives were measured in anhydrous acetonitrile containing
ca. 2.5 mM imide and 0.10 M tetrabutylammonium perchlorate
(TBAP) as a supporting electrolyte. Solutions were bubbled with
nitrogen prior to measurement. The cyclic voltammograms were
obtained using a BAS CV-1B CV controller that was interfaced
to a pentium PC for data acquisition. For these measurements,
platinum working and counter electrodes were employed, with
a silver/silver chloride (approximately 3 M KCl) reference
electrode. A scan rate of 200 mV/s was employed. For reference,
the half-wave potential of a 5 mM ferrocene solution was
measured and determined to be 0.443 V vs Ag/AgCl with the
electrode system employed.
N-Methyl-N′-butyl-4,4′-bipyridiniumyl-1,8-naphthalimide
(C4Wio) and N-Methyl-N′-pentyl-4,4′-bipyridiniumyl-1,8-naph-
thalimide (C5Wio). C4vio and C5vio were prepared as the
diiodide salts using procedures identical to those described
above. Analytically pure material was obtained after recrystal-
lization from methanol four times. 1H NMR (DMSO-d6):
(C4OTs) 8.41 (dd, 4H, naphth), 7.81 (t, 2H, naphth), 7.72 (d,
2H, ts), 7.36 (d, 2H, ts), 4.01 (m, 4H, CH2), 2.29 (s, 3H, CH3),
1.57 (m, 4H, CH2). 1H NMR (DMSO-d6): (C4vio) 9.39 (d, 2H,
vio), 9.29 (d, 2H, vio), 8.77 (dd, 4H, vio), 8.54 (dd, 4H, naphth),
7.92 (t, 2H, naphth), 4.75 (t, 2H, CH2), 4.47 (s, 3H, CH3), 4.16
(m, 2H, CH2), 2.11 (m, 2H, CH2), 1.75 (m, 2H, CH2). Anal.
Calcd (C27H25N3O2I2‚2H2O): C, 46.24; H, 3.59; N, 5.99. Found:
C, 46.58; H, 3.79; N, 6.04. 1H NMR (DMSO-d6: (C5OTs) 8.41
(d, 4H, naphth), 7.81 (t, 2H, naphth), 7.39 (d, 2H, ts), 6.84 (d,
2H, ts), 4.35 (t, 2H, CH2), 4.21 (t, 2H, CH2), 2.29 (s, 3H, CH3),
1.93 (m, 6H, CH2). 1H NMR (DMSO-d6): (C5vio) 9.39 (d, 2H,
vio), 9.31 (d, 2H, vio), 8.79 (dd, 4H, vio), 8.49 (dd, 4H, naphth),
Nanosecond transient absorption measurements were carried
out using 355 nm excitation. A detailed description of the laser
flash photolysis apparatus has been previously published.22
Results
A series of donor-acceptor diad systems (C2vio-C6vio),
containing 1,8-naphthalimide covalently attached to 4,4′-bipy-

Photoinduced ET in 1,8-Naphthalimide/Viologen Systems
J. Phys. Chem. A, Vol. 104, No. 29, 2000 6781
Figure 2. Ground-state UV absorption spectrum of C3py (triangles),
methylviologen (crosses), and C3vio (solid line, no symbol) measured
in phosphate buffer (10 mM; pH 7.0).
ridinium, was prepared as shown in Figure 1. The separation
distance between the two components was varied by increasing
the number of methylene spacers. The analogous series (C2py-
C6py) was synthesized as water-soluble “model” compounds
to be used in the photophysical studies.
Ground-State Spectral and Redox Studies. The UV spectra
of C3py and C3vio are shown in Figure 2. The ground-state
spectra of the other compounds (C2py-C6py) within the series
are identical. Likewise, the spectral properties of the naphthal-
imide-viologen diads (C2vio-C6vio) do not vary with the
number of intervening methylene units (data not shown). As
shown in Figure 2, the ground-state absorption spectra of the
covalently linked donor-acceptor systems is adequately de-
scribed as the sum of the 1,8-naphthalimide and viologen parts.
Specifically, there are no spectral perturbations in the 344 nm
absorption band of the naphthalimide moiety upon covalent
attachment of the viologen. Moreover, the additional absorption
intensity of C3vio relative to C3py, in the 200-300 nm range,
is attributed to the presence of 4,4′-bipyridinium.23 Thus, the
spectra indicate no evidence of electronic interaction in the
ground state.
Figure 3. Cyclic voltammograms of (a) C3py and (b) C3vio measured
in acetonitrile (0.10 M TBAP) vs Ag/AgCl (3 M KCl).
The redox properties of the compounds were characterized
by cyclic voltammetry. Voltammograms of C3py and C3vio
are shown in Figure 3a,b, respectively. In both cases, the
oxidation and reduction waves are completely reversible. From
Figure 3a, a single reduction wave was observed. The half-
wave potential (E1/2), taken as the midpoint of the cathodic
and anodic peak potentials, was determined to be -1.30 V vs
Ag/AgCl. At more negative potentials, solvent reduction
occurs. In Figure 3b, three reduction waves (E1/2 ) -0.39,
-0.83, and -1.32 V vs Ag/AgCl) are apparent. Within each of
the series, the half-wave potentials are identical within experi-
mental error ((10 mV). From the data shown in Figure 3, we
assign the reversible process occurring at -1.32 V as the one-
electron reduction of the naphthalimide moiety. This is nearly
identical to that reported for an uncharged 1,8-naphthalimide
derivative.22,24 By virtue of their absence in Figure 3a, we assign
the -0.39 and -0.83 V reduction waves to two successive
viologen-centered reduction processes. The reversible and
successive single electron reductions of viologen dications in
aqueous solution are known (Eo1/2 ) -0.45 and -0.88 V vs
NHE)25 and are virtually identical to the viologen-centered
reductions of C3vio.26 No redox processes were observed
up to a potential of 2.0 V, where solvent oxidation began to
occur.
Figure 4. Stern-Volmer quenching (fluorescence and inset of plot)
of C3py by MV2+. [MV2+] ) 0, 9.85, 19.7, 39.4, 59.1, 78.8, 98.5, 137
mM. All spectra were obtained in 0.5 M sodium phosphate buffer (pH
7.0) using an excitation wavelength of 344 nm.
Bimolecular Quenching of Naphthalimide Singlet Excited
States. As shown in Figure 4, the addition of methylviologen
to an aqueous solution of C3py results in quenching of the
naphthalimide fluorescence. Since the quenching process is a
diffusional one between two cationic species, a sufficiently high
buffer concentration (0.5 M sodium phosphate, pH 7.0) was
used to maintain a constant ionic strength of the solution as
methylviologen is added. At the highest methylviologen con-
centration employed (ca. 0.1 M), the ground-state absorption
spectrum of the naphthalimide was unperturbed and showed no
evidence of complex formation. The ratio of the integrated
fluorescence intensities before and after the addition of meth-
ylviologen (Io/I) was fit to eq 1. Linear regression analysis of
the data shown in the inset of Figure 4 yielded a Stern-Volmer

6782 J. Phys. Chem. A, Vol. 104, No. 29, 2000
Le et al.
1
Figure 5. Fluorescence decay of C3py* (5 µM in 10 mM pH 7.0
phosphate buffer) measured by time-correlated single-photon counting.
The instrument response function (dashed line) is shown, along with
the best reconvoluted fit of the data. An excitation wavelength of 344
nm was employed. The integrated emission between 370 and 500 nm
is shown.
Figure 6. Steady-state fluorescence spectra, of C3py (top, solid line),
C6vio, C5vio, C4vio, C3vio, and C2vio (in order of decreasing
intensities). Spectra were recorded using solutions of matched optical
densities at 344 nm in 10 mM phosphate buffer (pH 7.0).
constant (KSV) of 7.80 M-1
.
fluorescence intensity. However, the identical fluorescence
spectral features shown in Figures 4 and 6 suggests that there
is no electronic perturbation of naphthalimide by the covalently
attached viologen. The magnitude of the fluorescence quenching
increases with decreasing naphthalimide-viologen separation
distance. The mechanims of inter- and intramolecular quenching
by methylviologen, along with the distance-dependent fluores-
cence quenching in the covalently linked systems, are discussed
below.
Io
) 1 + KSV[MV2+]
(1)
I
Singlet-State Lifetimes. The singlet-state lifetime of one of
the pyridinium-substituted 1,8-naphthalimides was measured
using single-photon counting techniques. The fluorescence decay
1
of C3py* is shown in Figure 5, along with the instrument
response of the system. The decay is adequately fit to a single-
exponential decay model. From reconvolution analysis of this
decay, we estimate the lifetime of the S1 state to be 2.46 (
0.20 ns. This lifetime is nearly identical to a lifetime of 2.4 ns
for the decay of the S1-Sn absorption in picosecond pump-
probe experiments.22 Lifetime measurements of the other
compounds within the series were not made. However, since
the relative fluorescence quantum yields of C2py-C6py were
identical, it is reasonable to assume that the five compounds
have the same singlet-state lifetime.
From the measured singlet state lifetime, along with the
Stern-Volmer quenching results shown in Figure 4, a bimo-
lecular rate constant of 3.2 × 109 M-1 s-1 is obtained for
quenching of the naphthalimide singlet excited state by meth-
ylviologen in aqueous buffered solution. The mechanism of
quenching is addressed below.
Intramolecular Quenching of Naphthalimide Excited
Singlet States. Shown in Figure 6 (top spectrum) is the
fluorescence emission spectrum of C3py measured in buffered
aqueous solution. Using solutions of matched optical density
at 344 nm (the excitation wavelength), the emission spectra were
recorded for the other compounds in the pyridinium-substituted
1,8-naphthalimide series. Both the spectral shape and intensities
were identical to the top spectrum shown in Figure 4. For each
pair of “model” and viologen-linked compounds (C2py and
C2vio, C3py and C3vio, C4py and C4vio, C5py and C5vio,
C6py and C6vio), fluorescence emission spectra were acquired.
Within each pair, the optical densities at the excitation wave-
length were matched. Since the fluorescence quantum efficien-
cies were identical for each member of the pyridinium-linked
naphthalimide series, the intensities of the “model” compounds
were used to normalize the data within the series. The combined
data are shown in Figure 6. Apparent from Figure 6, the covalent
attachment of the 4,4′-bipyridinium moiety diminishes the
Discussion
Mechanisms of Inter- and Intramolecular Quenching.
From the data shown in Figure 4, it is apparent that the
methylviologen dication quenches the singlet excited state of
the pyridinium-functionalized 1,8-naphthalimides. When the
viologen and naphthalimide moieties are covalently attached,
quenching of the singlet excited state of the naphthalimide by
the 4,4′-bipyridinium also occurs. In both cases, we consider
two mechanisms of reactivity: (i) singlet-singlet energy transfer
from the electronically excited naphthalimide to methylviologen
and (ii) photoinduced electron transfer. The UV absorption
spectrum of methylviologen is well-known, and has a long-
wavelength onset of <350 nm.23 Fluorescence of the naphtha-
lene imide chromophores used in this study is observed only at
wavelengths longer than 350 nm (Figures 4 and 6). Since there
is no spectral overlap between the viologen absorption spectrum
and naphthalimide fluorescence spectrum, we conclude that
energy transfer from the S1 state of naphthalimide to methyl-
viologen is thermodynamically uphill and is not a likely
quenching pathway. We thus consider electron transfer quench-
ing processes.
It is known that both methylviologen and 1,8-naphthalimide
species can act as electron acceptors. The one-electron reduction
potential of methylviologen (E01/2 ) -0.450 V vs NHE)27 is
more positive than that of naphthalimide (E01/2 ) -1.26 V vs
SCE).16 There have been no published reports of viologen
or 1,8-naphthalmide as electron donors in photoinduced
charge transfer processes. The oxidation potential of 4-amino-
1,8-naphthalimide (1.20 V vs SCE) has been reported.28
However, oxidation of this chromophore is largely centered
on the amine substituent. This is supported by the fact that,
in this chromophore, the lowest excited singlet state is
largely charge transfer (amine to aromatic amide) in nature.29,30

Photoinduced ET in 1,8-Naphthalimide/Viologen Systems
J. Phys. Chem. A, Vol. 104, No. 29, 2000 6783
Figure 7. Transient absorption spectrum obtained upon 355 nm pulsed
excitation of C2py (9 µM) in the presence of 0.33 M methylviologen
in argon-saturated aqueous solution containing 0.50 M phosphate buffer
(pH 7.0). Spectra were recorded 0 (9), 12 (0), and 70 (b) µs after
excitation.
Figure 8. Rate constants for intramolecular electron transfer plotted
according to eq 6. Rate constants were evaluated using data from
Figures 5 and 6, along with eq 4 (ket ) 1.5 × 1010 s-1, 6.0 × 109 s-1
,
1.5 × 109 s-1, 3.8 × 108 s-1, and 8.3 × 107 s-1 for C2py-C6py).
The cyclic voltammograms measured in this study did not
reveal any oxidation processes up to 2.0 eV. Clearly, one of
the diad components must act as an electron donor for a
photoinduced electron transfer quenching mechanism to be
operative.
(V) is given by eq 3
∆G° ) -(E1/2(V2+/V•+) - E1/2(NI•+/NI)) - Eoo (3)
To elucidate the mechanism of singlet-state quenching, laser
flash photolysis experiments were carried out to identify the
quenching products. Shown in Figure 7 is the transient absorp-
tion spectrum observed upon pulsed 355 nm excitation of C2py
in the presence of methylviologen. Under the conditions
employed, 72% of the C2py singlet states were quenched by
methylviologen and all of the light is absorbed by the naph-
thalimide chromophore. Immediately after the 8 ns laser pulse,
spectral features characteristic of both viologen radical cation
(395 and 600 nm)23 are superimposed with the T1-Tn naph-
thalimide excited state (λmax ) 480 nm).18 Since the spectral
features of the singlet-derived product are consistent with
production of viologen radical cation, we conclude that quench-
ing of the naphthalimide singlet excited state is via electron
transfer to methylviologen (eq 2a).
where Eoo is the singlet-state energy of the naphthalimide
chromophore. The S1 energy of N-substituted 1,8-naphthalimide
has been determined by ourselves and others to be in the range
3.4-3.5 eV.22,31 Using Eoo ) 3.4 eV, along with the V2+/V•+
half-wave potential of -0.39 V vs Ag/AgCl measured in this
work, it is apparent that exergonic electron transfer will occur
when the oxidation potential (E1/2(NI•+/NI)) is less than ca. 3.0
eV. Solvent oxidation precludes measuring potentials greater
than 2.0 eV (Figure 3). Thus, it is not surprising that the
oxidation wave was not observed but that photoinduced electron
transfer from methylviologen to the naphthalimide excited state
does occur.
Distance Dependence of Electron Transfer Rates. From
the singlet-state lifetimes of the “parent” naphthalimide com-
pounds (C2py-C6py), along with the steady-state fluorescence
spectra shown in Figure 6, the rate constants for intramolecular
electron transfer were calculated (eq 4).
1Cxpy* + MV2+ f 1[Cxpy•+‚‚‚MV•+]
1[Cxpy•+‚‚‚MV•+] f Cxpy•+ + MV•+
1[Cxpy•+‚‚‚MV•+] f Cxpy + MV
kq (2a)
ksep (2b)
I
kET
kd
o ) 1 +
(4)
I
krec
(2c)
In eq 4, Io and I are the integrated fluorescence intensities for
each pair of “parent” and viologen-linked compounds, respec-
From the spectra, we estimate the efficiency of production
of solvent-separated redox products ksep/(ksep + krec) to be 12%.
On longer time scales, additional viologen radical cation is
produced from quenching of the triplet state.
By analogy with the intermolecular quenching products
observed, we attribute fluorescence quenching of naphthalimide
by the covalently attached viologen to electron transfer. No
significant amounts of long-lived intramolecular quenching
products were observed on time scales longer than 10 ns.
However, in all cases, a small and reproducible absorption
feature at 395 nm, characteristic of the viologen radical cation,
is seen in the spectra.
tively. The intramolecular electron transfer rate constants, kET
,
was evaluated using the known singlet-state lifetimes of the
“parent” Cxvio compounds (kd ) 1/τd ) 4.1 × 108 s-1).32 The
rate constants are given in Figure 8.
As shown in Figure 8, the rate constants vary exponentially
with the number of intervening methylenes. In a classical
treatment, kET is related to the electronic coupling matrix element
between donor and acceptor (|V|), the Gibbs free energy change
for the electron transfer step (∆G°), and the sum of the molecular
(λi) and solvent (λs) reorganization energies (λ ) λi + λs)
according to eq 5.33
Energetics of Electron Transfer. Neglecting solvation of
the charge transfer products relative to the reactants, the
thermodynamic driving force for electron transfer from the
singlet excited state of naphthalimide (NI) to methylviologen
-(∆Go + λ )2
4λRT
1/2
2
2|V| π3
kET
)
exp
(5)
(
)(
)
(
)
h
λRT

6784 J. Phys. Chem. A, Vol. 104, No. 29, 2000
Le et al.
2
W. J. Phys. Chem. 1988, 92, 6958-6962. Oliver, A. M.; Craig, D. C.;
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The |V| term decreases exponentially with distance (eq 6),
where â describes the efficiency of the intervening medium to
conduct the electron and rDA is the donor/acceptor separation
distance.
2
|V|
exp(-â (rDA - roDA
)
(6)
The exponential distance dependence of kET shown in Figure
8 supports an electron transfer mechanism involving superex-
change through the polymethylene spacer. From a linear
regression analysis of the data shown in Figure 8, the â value
was calculated to be 1.32 per methylene or 1.04 Å-1 (assuming
1.27 Å per methylene).34 The fact that kET decays exponentially
over the range of distances provided by 2-6 methylenes
(9) Yonemura, H.; Kasahara, M.; Saito, H.; Nakamura, H.; Matsuo,
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suggests that conformation has a relatively small effect on kET
.
Yonemoto et al. have studied the intramolecular electron transfer
kinetics in covalently linked tris(bipyridyl)ruthenium(II)/violo-
gen systems.8 In this work, exponential distance dependences
up to 6-methylene linkages were observed, suggesting that the
kinetics are conformation-independent. In the tris(bipyridyl)-
ruthenium(II)/viologen systems, an apparent â value (uncor-
rected for solvent reorganization energies) of 1.38 Å-1 was
obtained for forward electron transfer in acetonitrile. For longer
linkages (7-8 methylenes), rate constants deviated from the
predicted distance dependence, suggesting that an additional ET
pathway is provided by through-space interactions. For these
longer-chain systems, complexation by â-cyclodextrin brought
the kinetic data points back onto the predicted line.8 In a separate
study, Park et al. demonstrated exponential distance dependences
for electron transfer in aqueous solutions of covalently linked
naphthoxyl/viologen donor/acceptor systems.7 In this study, an
apparent â value of 0.86 Å-1 was obtained in aqueous solution.
Thus, our observation of predominantly an exponential decrease
in kET with distance (Figure 8), coupled with previously
published reports, suggests that through-space interactions
between the electron donor and acceptor, via conformational
flexibility of the polymethylene linkage, have little effect on
the electron transfer process. Although the apparent â value of
1.04 Å-1 obtained in our polymethylene-linked naphthalimide/
viologen diads is not identical to those reported previously,7,8
it is in the range (1.38-0.86 Å-1) of those observed for other
donor/acceptor systems.
(10) Toki, A.; Yonemura, H.; Matsuo, T. Bull. Chem. Soc. Jpn. 1993,
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3861.
In conclusion, we have demonstrated singlet state quenching
of N-substituted 1,8-naphthalimide by methylviologen. The
transient absorption spectrum of the intermolecular electron
transfer products is consistent with production of reduced
methylviologen, suggesting that photooxidation of the naph-
thalimide excited state is occurring. When the electron donor
and acceptor are covalently linked by a series of 2-6 methylene
spacers, a linkage-dependent quenching occurs. The rate constant
for electron transfer decays exponentially with distance, con-
sistent with electron transfer mediated by the polymethylene
bridge. From the results, an apparent â of 1.04 Å-1 is obtained.
(23) Watanabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617-2619.
(24) The Fc+/Fc redox couple has been measured in acetonitrile: Eo1/2
) 0.69 V vs NHE (Barrette, W.; Johnson, H.; Sawyer, D. Anal. Chem.
1984, 56, 1890). Using this value, along with the measured Fc+/Fc half-
wave potential in this work (0.43 V vs Ag/AgCl), the half-wave potential
for the naphthalimide-centered reduction in C3vio is computed to be -1.07
V vs NHE in acetonitrile. This is nearly identical to the value published in
ref 22 (Eo1/2 ) -1.01 V vs NHE).
(25) Bird, C. L.; Kuhn, A. T. Chem. Soc. ReV. 1981, 10, 49-82.
(26) Using Eo1/2 ) 0.40 V vs NHE for the Fc+/Fc couple in aqueous
solution (Sawyer, D. T.; Sobkowiak, A.; Roberts, J. I. Electrochemistry for
Chemists, 2nd ed.; Wiley-Interscience: New York, 1995; p 203), along
with our measured half-wave potential for the ferrocene/ferrocenium couple
(0.43 V vs Ag/AgCl), we estimate the viologen-centered reductions to be
-0.42 and -0.86 V vs NHE.
Acknowledgment. This work is supported by the American
Cancer Society, Maryland Division.
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Photoinduced ET in 1,8-Naphthalimide/Viologen Systems
J. Phys. Chem. A, Vol. 104, No. 29, 2000 6785
(32) The fluorescence lifetime was measured for C3py (Figure 5). Since
the fluorescence quantum yields of C2py-C6py were identical, a singlet-
state lifetime of 2.46 ns was used for all of the compounds within the Cxpy
series.
(33) Newton, M. D.; Sutin, N. Annu. ReV. Phys. Chem. 1984, 35, 437-
480. Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265-
322. Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701.
(34) The distance dependence is referred to as an “apparent” â since it
neglects solvent reorganization energies (λs). Solvent reorganization energies
have been calculated for electron donor/acceptor systems separated by 1-5
and 7 or 8 methylene units (ref 11). If the donor and acceptor are considered
as hard spheres 3.1 Å radii, λs were found to range from 1.08 to 1.23 Å for
spheres separated by 2-5 methylenes. Thus, it is assumed that deviation
from the predicted exponential dependence (eq 5) is negligible.