A rigid chlorin-naphthalene diimide conjugate. A possible new noncovalent electron transfer model system

Article abstract of DOI:10.1021/jo9810112

Described in this paper is the synthesis and study of a rigid "coplanar" noncovalent electron-transfer model system. This putative noncovalent complex juxtaposes a novel donor (chlorin) and acceptor (naphthalene diimide) via a three-point hydrogen bonding interaction (CDCl₃, K_a = 364 ± 47 M^-1). It was studied by steady state fluorescence, time-resolved luminescence, and transient absorption methods. The results of the studies are consistent with (1) forward intraensemble electron transfer (ET) taking place rapidly following photoexcitation of the chlorin donor at 575 nm (k_ET = 7.6 × 10⁸ s^-1; ΔG_cs ～ -457 mV; Φ = 0.91) and (2) back electron transfer occurring even more rapidly.

Full text of DOI:10.1021/jo9810112

7370
J . Org. Chem. 1998, 63, 7370-7374
A Rigid Ch lor in -Na p h th a len e Diim id e Con ju ga te. A P ossible New
Non cova len t Electr on Tr a n sfer Mod el System
J onathan L. Sessler,* Christopher T. Brown,† Don O’Connor, Stacy L. Springs,‡
Ruizheng Wang,§ Muhunthan Sathiosatham, and Takuji Hirose|
Department of Chemistry and Biochemistry, University of Texas at Austin, Texas 78712
Received May 28, 1998
Described in this paper is the synthesis and study of a rigid “coplanar” noncovalent electron-transfer
model system. This putative noncovalent complex juxtaposes a novel donor (chlorin) and acceptor
(naphthalene diimide) via a three-point hydrogen bonding interaction (CDCl3, Ka ) 364 ( 47 M-1).
It was studied by steady state fluorescence, time-resolved luminescence, and transient absorption
methods. The results of the studies are consistent with (1) forward intraensemble electron transfer
(ET) taking place rapidly following photoexcitation of the chlorin donor at 575 nm (kET ) 7.6 × 108
s-1; ∆Gcs ∼ -457 mV; Φ ) 0.91) and (2) back electron transfer occurring even more rapidly.
Sch em e 1
In tr od u ction
Long-range biological electron transfer reactions in
proteins are of fundamental importance and play critical
roles in such ubiquitous processes as photosynthesis and
oxidative phosphorylation.1 Despite this importance, a
detailed understanding of these reactions remains elu-
sive. For instance, considerable effort continues to be
devoted to understanding how specific pathways linking
a donor and an acceptor, serve to mediate the overall
electron transfer (ET) process.2 One way in which this
critical issue is being addressed is through the synthesis
and study of simple, noncovalently constructed model
systems.3-6 Here, the overriding objective has been to
* To whom inquiries should be addressed. E-mail: sessler@
mail.utexas.edu. Phone: (512) 471-5009. Fax: (512) 471-7550.
† Current address: Eastman Kodak Co., 1999 Lake Ave., Building
82, RL Rochester, NY 14650-2103. E-mail: cbrown@kodak.com.
‡ Current address: Department of Chemistry, Princeton University,
Princeton, NJ 08544. ssprings@princeton.edu.
determine whether pathways containing hydrogen bonds
or other noncovalent contacts are competitive (in terms
of allowable ET rates) with those containing only σ-bonds.7
Unfortunately, even in the context of model systems, this
is a point that remains contentious.
In this work the synthesis of a new, rigid noncovalent
model system (ensemble I, Scheme 1) is described. It
employs chlorin 1 as the singlet excited-state electron
donor and the diimide 2 as the acceptor. The donor and
acceptor are juxtaposed by a three-point hydrogen bond-
§ Current address: Room 3A, J ames Graham Brown Cancer Center,
529 South J ackson St., Louisville, KY 40202. Ruizwang@hotmail.com.
| Current address: Department of Applied Chemistry, Faculty of
Engineering, Saitama University, Shimo-Oukubo 338 Urawa, Saitama,
J apan. hirose@apc.saitama-u.ac.jp.
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S0022-3263(98)01011-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/30/1998

A Rigid-Chlorin-Naphthalene Diimide Conjugate
J . Org. Chem., Vol. 63, No. 21, 1998 7371
F igu r e 1. Overlay of 5,10,15,20-tetraphenyl-21H,23H-porphyrin [s, 1.14 × 10-6 M] and (1) [- - -, 2.59 × 10-6 M] in toluene (left
trace) and overlay of the fluorescence profile of 5,10,15,20-tetraphenyl-21H,23H-porphyrin (H2TPP) [s, 2.41 × 10-4 M] and (1)
[- - -, 2.27 × 10-4 M] in toluene (right trace).
Sch em e 2
ing interaction which is designed to stabilize a “coplanar”
supramolecular complex (ensemble I) with a ca. 7 Å edge-
to-edge distance between the redox active chlorin and
diimide components. Irradiation of this complex at 573
nm gives rise to spectroscopic changes that are readily
teristics of 1 were recorded and compared to those of
tetraphenylporphyrin (H2TPP). As can be seen from an
inspection of Figure 1, there is a large bathochromic shift
in the absorption spectra of 1 versus H2TPP with the
greatest difference being manifest in terms of a ca. 20
nm red-shift in the Soret absorption (λmax values of 439
and 419 nm were recorded for 1 and H2TPP, respectively).
The emission profiles of these two species also differ
somewhat as illustrated in Figure 1.
interpretable in terms of rapid (ket ) 7.6 × 108 s-1
)
forward electron transfer followed by even faster charge
recombination (back ET).
Evidence for the formation of complex I in CDCl3 came
from 1H NMR spectroscopic studies. Specifically, as
illustrated in Figure 2, analysis of the downfield shift for
the diimide imine proton as a function of increasing
chlorin (1) concentration provided support for a 1:1
binding model and yielded an association constant Ka of
Resu lts a n d Discu ssion
The preparation of the key monomers 1 and 2 is
summarized in Scheme 2. Briefly, chlorin 1 was pre-
pared in 85% yield by condensing commercially available
2,4,5,6-tetraaminopyrimidine with 3 (17,18-dioxotetra-
phenylchlorin8) in pyridine at reflux. Acceptor 2, on the
other hand, was obtained in two steps. First, intermedi-
ate 4 was generated in 28% yield by condensing 2,5-di-
tert-butylaniline with 1,4:5,8-naphthalenetetracarboxylic
acid dianhydride.2 This latter material was then con-
verted to the diimide 2 by treating with urea in boiling
1,3,5-trichlorobenzene.6
364 ( 47 M-1 10
While such findings are consistent with
.
expectations, diimide compounds are notorious for π-stack-
ing in solution.11 Therefore, to alleviate concerns that
such stacking could be taking place in the present
instance, the aromatic naphthalene protons of 2 were also
1
monitored during the course of the H NMR titrations.
In point of fact, no discernible changes in these latter
To the best of our knowledge, the present paper is the
first in which porphyrins/chlorins such as 1 have been
studied in the context of noncovalent ET studies.9 Ac-
cordingly, the absorption and emission spectral charac-
(9) Mention has previously been made (in the form of unpublished
results) for compounds similar to 1. Peenfield, K. W.; Miller, J . R.;
Paddon-Row: M. N.; Cotsaris, E.; Oliver, A. M. J . Am. Chem. Soc. 1987,
109, 5061-5065.
(10) Kra´l, V.; Furuta, H.; Shreder, K.; Lynch, V.; Sessler, J . L. J .
Am. Chem. Soc. 1996, 118, 1595-1607.
(11) Lokey, S. R.; Iverson, B. L. Nature 1995, 375, 303-305.
(8) Crossley, M. J .; P. L. Burn, Langford, S. J .; Prasher, J . K. J .
Chem. Soc., Chem. Commun. 1995, 1921-1923, and references therein.

7372 J . Org. Chem., Vol. 63, No. 21, 1998
Sessler et al.
linear fashion consistent with diffusional “Stern-Volmer”
quenching. On the basis of these results and previous
precedent,5 the shorter lifetime (τ1; 1.2 ns) is assigned to
1 bound in complex I, and the longer lifetime (τ2; 13 ns)
is attributed to the free chlorin 1.
To the extent the above assignments are correct, the
most likely explanation for the observed effects is that
rapid intracomplex photoinduced electron transfer occurs
from the singlet excited state of 1 to the diimide acceptor
2 within assembly I. The rate constant for this electron-
transfer reaction (∆Gcs = -457 mV)12 was determined to
be ca. 7.6 × 108 s-1 with a quantum yield of 0.91 (Φ )
τ1kET).5,13
Similar experiments were also performed in dichloro-
methane and yielded analogous results. In particular,
the lifetime (but not the amplitude) of the short compo-
nent was found to remain constant during a titration in
which the concentration of 2 was steadily increased from
0 to 1.23 × 10-2 M. Thus, from the lifetime recorded (0.31
ns), a rate constant for intraensemble ET (kET) of ca. 3.1
× 109 s-1 with a quantum yield, (Φ ) τ1kET), of 0.97 could
be calculated.13
F igu r e 2. 1H NMR binding isotherm for 1 with 2 in CDCl3
at 25 °C. Nonlinear least-squares analysis, carried out in
accord with methods described preciously,10 gave an associa-
tion constant Ka ) 364 ( 47 M-1
.
signals were seen (as a function of concentration). This
is interpreted in terms of the extent of π stacking between
1 and 2 being small or nonexistent under the conditions
of the experiment.
While the single-photon counting experiments provide
evidence consistent with electron transfer taking place
rapidly within the ensemble I, it is important to appreci-
ate that this evidence is only circumstantial. Unfortu-
nately, all efforts to observe a diimide-derived radical
anion14 using time-resolved spectroscopy met with failure.
As detailed below, this failure does not reflect the fact
that species 2 cannot be reduced to the radical anion.
Rather, it is rationalized in terms of the back ET reaction
(2-• + 1+• f 2 + 1) being so fast in toluene or dichloro-
methane that no appreciable concentration of the charge
separated species (2-• + 1+•) ever builds up to an
observable level (eq 1).
Steady-state fluorescence quenching experiments in-
dicated that the fluorescence of 1 is quenched to a greater
extent by 2 when compared to the case where 1 was
replaced by H2TPP. This difference leads us to propose
that the hydrogen bond derived recognition site of 1
allows for the ready formation of the putative complex I
and, as a result, more efficient ET-based quenching of
the chlorin singlet excited state. Addition of hydrogen
bond competitors (i.e. methanol, uracil) served to restore
the fluorescence of 1.
Single-photon counting experiments performed in tolu-
ene further served to support a model where the forma-
tion of complex I allows for rapid intraensemble ET. The
chlorin 1 is similar to H2TPP in that, in the absence of a
quencher, its emission profile is monoexponential with
a lifetime of 13.2 ns. Addition of the chlorin excited-state
quencher, diimide 2, resulted in the observation of a
biexponential emission decay profile for 1 (excitation
wavelength, 573 nm and emission monitored at 640 nm).
Under these conditions, it was found that the amplitude
(i.e. preexponential portion), but not the magnitude, of
the fast component lifetime (τ1; 1.2 ns) increased as the
concentration of the quencher (2) was increased. Simi-
larly, the amplitude of the slower component (τ2; ca. 13
ns) was found to decrease as a function of increasing
quencher concentration. Analysis of the relevant single
photon counting data in a manner analogous to that
previously described5 yielded an association constant of
663 M-1 (toluene) for the formation of complex I. This
value is of a comparable magnitude to that determined
in CDCl3 (Ka ) 364 ( 47 M-1), a more polar solvent, using
1* + 2 charge separation8 1-• + 2+•
1-•+ 2+• charge recombination8 1 + 2
(1)
Evidence that a radical anion of 2 may be formed under
appropriate conditions came from spectroelectrochemical
analyses. Using such methods it was found that the
diimide 2 exhibits virtually no light absorption between
400 and 700 nm. However, reduction of 2 with a single
electron provides a species, 2-•, with a λmax of 476 nm (ꢀ
∼ 104 M cm-1, cf. Figure 3) that, in principle, could
provide a very diagnostic “handle” for observing a photo-
induced chlorin-diimide ET event, provided the rate of
charge recombination (back ET) is such that an ap-
preciable concentration is allowed to build up.
(12) The excited-state energy for 1 was estimated to be approxi-
mately 1.95 eV from the intersection of the absorption and emission
spectra, cf. Figure 1. To determine the ground-state oxidation potential
for 1 (+926 mV) and the ground-state reduction potential for 2 (-567
mV), cyclic voltammetry experiments were performed in dichloro-
methane/TBAPF6 (Ag/AgCl, Pt electrode). For the forward ET reaction
in I, a driving force of -457 mV was calculated using the following
equation:
1
the H NMR based methods discussed above.
In analogy to what was seen under steady-state condi-
tions, disruption of the noncovalent complex I by addition
of a hydrogen bond competitor (e.g., methanol or uracil)
restored the monoexponential fluorescence decay profile
of 1. Additionally, titration of the porphyrin with a
diimide acceptor lacking the uracil-type hydrogen bond-
ing recognition site (i.e. N,N′-diphenyl-1,5:4,8-tetracar-
boxynaphthalenedicarboximide) did not alter the basic
monoexponential nature of the fluorescence decay profile
typically observed for 1. Instead, under these conditions
the monoexponential decay was found to decrease in a
∆GCS ) -E1/2(A-/A) - ((E0,0) - E1/2(D/D+))
(13) Calculated using the expression: k ) 1/τ2 - 1/τ1.
(14) Osuka, A.; Nakajima, S.; Okada, T.; Taniguchi, S.; Nozaki, K.;
Ohno, T.; Yamazaki, I.; Nishimaura, Y.; Mataga, N. Angew. Chem.,
Int. Ed. Engl 1996, 35, 92-95, and references therein.
(15) Sessler, J . L.; Lisowski, J .; Boudreaux, K. A.; Lynch, V.; Barry,
J .; Kodadek, T. J . J . Org. Chem. 1995, 60, 5975-5978.

A Rigid-Chlorin-Naphthalene Diimide Conjugate
J . Org. Chem., Vol. 63, No. 21, 1998 7373
charge recombination (back ET) occurs even more quickly
than the original charge separation process. On the basis
of the steady-state fluorescence and single-photon count-
ing studies described above, we favor the latter inter-
pretation.
Con clu sion
This present paper outlines a new approach to the
construction of rigid noncovalent electron-transfer en-
sembles. While the present prototypic ensemble works
well in terms of allowing photoinduced ET, it falls short
in terms of establishing a long-lived charge separated
state. Current efforts are therefore focused on the
construction of alternative systems, containing different
donor-acceptor pairs, that might allow the presumed
radical cation-radical anion pair to be better observed.
F igu r e 3. Spectroelectrochemical absorption spectrum for 2
(1.59 × 10-4 M) in anhydrous argon-degassed dichloromethane
(25 °C) with 0.15 M TBAPF6 as the electrolyte.
Exp er im en ta l Section
Gen er a l Meth od s.15 Proton NMR binding studies were
performed using titrations carried out in accord with previ-
ously reported methods.10 In these titrations the relevant
binding isotherms were obtained by monitoring the downfield
shift of the imide proton in 2, (initial concentration, 1.62 mM)
as the concentration of 1 was varied from 0 to 7.2 mM.
Analysis by nonlinear least-squares fitting procedures10 con-
firmed the proposed 1:1 binding stoichiometry and yielded an
association constant of 364 ( 47 M-1
.
Fluorescence lifetimes were measured by time-correlated
single-photon counting using a mode-locked, synchronously
pumped, cavity-dumped Rhodamine 6G dye laser. The excita-
tion wavelength used for the present chlorin-based systems
was at 573 nm while the emission was monitored at a variety
of wavelengths between 600 and 700 nm.
Transient absorption studies were performed with a mode-
locked, frequency-doubled Nd:YAG laser (pulse width 30 ps).
Excitation was at 532 nm.
F igu r e 4. Picosecond transient absorption data for I in
anhydrous argon-degassed toluene. Concentrations of chlorin
(1) and quencher (2) were 4.84 × 10-5 M and 2.7 × 10-3 M,
respectively. Excitation was effected at 532 nm.
In the hope that back ET reaction would be slow
enough to allow the radical anion of 2 to be observed,
picosecond transient absorption experiments involving I
were performed. Figure 4 shows transient absorption
spectra recorded for I following excitation with a 532 nm
laser pulse. As can be appreciated from an inspection of
this figure, there is no evidence for an appreciable
concentration of the reduced diimide species (2-•). In fact,
the spectra recorded for I are identical in all respects to
those obtained using pure 1, with the notable exception
that the decay of the singlet excited state for 1 in
ensemble I is faster. Presumably, this latter result
simply reflects the fact that (forward) intraensemble ET
is both favorable and fast.
Ma ter ia ls. Toluene was distilled from sodium. Pyridine
and dichloromethane were distilled from calcium hydride.
Anhydrous N,N-dimethylformamide, dichloromethane, tetra-
aminopyrimidine, 1,4:5,8-naphthalenetetracarboxylic acid di-
anhydride, and tetraphenylporphyrin were purchased from
Aldrich Chemical Co. (Sure-Seal).
8,11,14,17-Tetr a k is(3,5-d i-ter t-bu tylp h en yl)-2,4-d ia m i-
n op ter id in o[2,3-b]p or p h yr in (1). A 50 mL round-bottom
flask was charged with 38 (400 mg, 0.365 mmol), 2,4,5,6-
tetraaminopyrimidine sulfate (174 mg, 0.73 mmol), and 25 mL
of anhydrous pyridine. This mixture was heated at reflux
under an argon atmosphere for 24 h. The pyridine was then
removed in vacuo. The resulting residue was taken up in
chloroform and washed 2 × 100 mL of water. The organic
layer was dried over Na2SO4. After filtration and removal of
solvent, this brown material was taken up in a minimum of
Analysis of the decay traces of Figure 4 at a variety of
wavelengths where both the excited chlorin and putative
anion absorb revealed a monoexponential decay with an
excited-state lifetime of 1.4 ns that is in excellent
agreement with the fast τ1 observed (for chlorin 1 in
ensemble I) by single-photon counting methods (vide
supra). A slow component, corresponding to τ2 in the
single-photon counting experiments, is also observed. If
an observable population of reduced diimide 2 were
present then one would have expected a more complex
transient absorption decay profile for the early time
events. Such complexity should be manifest at 476 nm
where the strongest anion-ascribable absorption feature
is found. However, even at this wavelength no kinetic
(i.e. nonexponential) or spectral complexity is observed.
It thus becomes necessary to assume that either forward
electron transfer is not occurring or that subsequent
chloroform and loaded onto a silica gel:hexanes column.
A
brown product was eluted off the column using 50% ethyl
acetate/hexanes (v/v) (TLC-silica gel, Rf ) 0.43, 5% MeOH,
CHCl3 (v:v)) to yield 371 mg (85%) of 1. The purified material
obtained in this way was recrystallized by allowing the
benzene to evaporate off from a solution containing the chlorin
product in a mixture of benzene/acetonitrile. This yielded 1
in the form of small brown needles. 1H NMR (250 MHz,
CDCl3): δ -2.54 (2H, br s), 1.50-1.55 (72H, m), 5.20-5.35
(2H, br s), 6.35-6.45 (2H, br s), 7.79 (2H, t (J ) 1.75 Hz)),
7.88 (1H, t (J ) 1.79 Hz)), 7.97 (1H, t (J ) 1.75 Hz)), 8.04 (2H,
d (J ) 1.79 Hz)), 8.07 (2H, d (J ) 1.77 Hz)), (2H, d (J ) 1.77
Hz)), 8.08 (2H, d (J ) 1.78 Hz)), 8.75-8.78 (3H, m), 8.93 (1H,
d (J ) 4.95 Hz)), 9.00 (2H, s) ppm. 13C NMR (250 MHz,
CDCl3): δ 31.74, 31.88, 35.17, 35.23, 116.91, 119.62, 120.23,
120.40, 120.45, 121.12, 121.16, 122.31, 123.13, 128.05, 128.14,
128.24, 128.48, 128.57, 129.45, 129.60, 129.81, 134.06, 134.58,
134.61, 137.95, 138.54, 139.26, 140.33, 140.85, 140.87, 141.19,
142.07, 144.45, 144.98, 147.14,148.76, 148.89, 149.36, 154.74,

7374 J . Org. Chem., Vol. 63, No. 21, 1998
Sessler et al.
155.45, 157.91, 162.82, 163.81 ppm. Mass Spectrum (FAB):
m/z (relative intensity) 1198 (M+, 100). Exact mass (FAB) for
C80H96N10: calcd 1197.7839, found 1197.7854. Anal. Calcd
for C80H96N10: C, 80.23; H, 8.08; N, 11.69. Found: C, 80.20;
H, 7.98; N, 11.64. λmax (ꢀ) 438.5 (221000 M-1 cm-1), 527.5
(19847 M-1 cm-1), 564 (5930 M-1 cm-1), 599 (9387 M-1 cm-1),
651 (589 M-1 cm-1) nm. Cyclic voltammetric measurement:
(0.15 M TBAPF6 in CH2Cl2; Pt working electrode; Ag/AgCl
reference; Pt wire counter electrode; sweep rate of 150 mV/s);
E1/2 ) +926 mV, E1/2 ) +1061 mV.
N-(2,5-Di-ter t-bu tylp h en yl)-1,5-d ica r boxy-4,8-n a p h th a -
len ed ica r boxim id e An h yd r id e (4). A 500 mL round-bottom
flask was charged with 1,4:5,8-naphthalenetetracarboxylic acid
dianhydride (19.44 g, 73 mmol) and 100 mL of anhydrous N,N-
dimethylformamide (DMF) and stirred under an argon atmo-
sphere. After warming to 80 °C, 2,5-di-tert-butylaniline (5 g,
24.3 mmol) in 50 mL of anhydrous DMF was added dropwise
over 30 min. The mixture was stirred at 80 °C for an
additional 12 h. The DMF was removed under high vacuum.
The resulting residue was suspended in 50 mL of CH2Cl2 and
loaded onto a silica gel:hexanes column. Byproducts were
removed by eluting with 10% ethyl acetate/hexanes (v/v). The
product was then eluted off using 50% ethyl acetate/hexanes
(v/v). This gave 3.06 g (28% yield) of 4. 1H NMR (250 MHz,
CDCl3): δ 1.25 (9H, s), 1.33 (9H, s), 7.03 (1H, d (J ) 2.16 Hz)),
7.53 (1H, dd (J ) 8.47, 2.20 Hz)), 7.64 (1H, dd (J ) 8.54 Hz)),
8.86 (4H, s) ppm. 13C NMR (63 MHz, CDCl3): δ 31.29, 31.83,
123.48, 126.97, 127.67, 127.91, 128.52, 129.48, 131.87, 133.56,
144.48, 151.04, 159.40, 163.76 ppm. Mass spectrum CIMS
(CH4): m/z (relative intensity, %) 456 [M + 1, 100]; HRCIMS:
for C28H26NO5 cald: 456.1811, found: 456.1806. Cyclic vol-
tammetric measurement: (0.15 M TBAPF6 in CH2Cl2; Pt
working electrode; Ag/AgCl reference; Pt wire counter elec-
trode; sweep rate of 150 mV/s); E1/2 ) -455 mV, E1/2 ) -927
mV.
reflux under an argon atmosphere for 1 h and then cooled to
room temperature. The trichlorobenzene was removed in
vacuo. The residue was taken up in 100 mL of chloroform and
washed with 100 mL of 10% NaHCO3 followed by 2 × 100 mL
of water. The organic layer was dried over Na2SO4, filtered,
and evaporated to dryness. The resulting crude material was
loaded onto a 50% ethyl acetate/hexanes (v/v) silica gel column
and eluted using the same solvent mixture. After collection
of the appropriate fractions and removal of solvent, a light-
yellow powder was obtained and recrystallized from cold
toluene to give 2 in the form of yellow-feathery needles (248
mg, 83%). 1H NMR (250 MHz, CDCl3): δ 1.27 (9H, s), 1.32
(9H, s), 6.99-7.00 (1H, dd (J ) 2.17 Hz)), 7.48-7.52 (1H, dd
(J ) 2.20, 8.51 Hz)), 7.60 (1H, d (J ) 8.54 Hz)), 8.38 (4H, q (J
) 7.61 Hz)), 8.95 (1H, s) ppm. 13C NMR (63 MHz, CDCl3): δ
31.19, 31.74, 34.28, 35.57, 126.69, 127.53, 128.99, 130.89,
131.40, 131.97, 143.73, 150.40, 162.44, 163.64 ppm. Mass
spectrum FAB-MS (NBA, 70 eV): m/z (relative intensity, %)
455 [M + 1, 100]; HRCIMS: for C28H27N2O4: calcd 455.1971,
found: 455.1968. Anal. Calcd for C28H26N2O4: C, 73.99; H,
5.77; N, 6.16. Found: C, 73.86; H, 6.03; N, 6.02. Cyclic
voltammetric measurement: (0.15 M TBAPF6 in CH2Cl2; Pt
working electrode; Ag/AgCl reference; Pt wire counter elec-
trode; sweep rate of 200 mV/s); E1/2 ) -567 mV, E1/2 ) -985
mV.
Ack n ow led gm en t. This work was supported by the
NIH (GM 41657) and the R. A. Welch Foundation (F-
1018).
Su p p or tin g In for m a tion Ava ila ble: Proton NMR chemi-
cal shift data for the aromatic protons of 2; steady-state
fluorescence data for quenching of 1 by 2; 1H NMR spectra for
1 and 2; single photon counting and transient absorption data
for I (11 pages). This material is contained in libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
N-(2,5-Di-ter t-bu tylph en yl)-1,5-dicar boxy-N′-h ydr ogen -
4,8-d ica r boxyn a p h th a len ed iim id e (2). A 25 mL round-
bottom flask was charged with 4 (300 mg, 0.659 mmol), urea
(1 g, 16.7 mmol), and 10 mL of 1,3,5-trichlorobenzene (freshly
distilled from CaH2). This reaction mixture was heated at
J O9810112