Long-lived triplet state charge separation in novel piperidine-bridged donor - Acceptor systems

Article abstract of DOI:10.1021/ja960980g

Two bichromophoric systems are presented that contain an N-alkylnaphthalimide electron acceptor and a 4-methoxyaniline (3a) or an aniline (3b) electron donor, respectively. Upon photoexcitation of 3a in cyclohexane electron transfer occurs in the singlet

Full text of DOI:10.1021/ja960980g

J. Am. Chem. Soc. 1996, 118, 8425-8432
8425
Long-Lived Triplet State Charge Separation in Novel
Piperidine-Bridged Donor-Acceptor Systems
†
†
‡
†
Saskia I. van Dijk, Cornelis P. Groen, Franti sˇ ek Hartl, Albert M. Brouwer, and
Jan W. Verhoeven*,†
Contribution from the Laboratory of Organic Chemistry, UniVersity of Amsterdam, Nieuwe
Achtergracht 129, 1018 WS Amsterdam, The Netherlands, and Laboratory of Inorganic
Chemistry, UniVersity of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The
Netherlands
X
ReceiVed March 25, 1996
Abstract: Two bichromophoric systems are presented that contain an N-alkylnaphthalimide electron acceptor and a
4
-methoxyaniline (3a) or an aniline (3b) electron donor, respectively. Upon photoexcitation of 3a in cyclohexane
1
+
-
electron transfer occurs in the singlet manifold to afford the short-lived (τf ) 0.75 ns) (D -A ) state in ca. 70%
+
-
yield. An important decay pathway of this D -A state consists of intersystem crossing (ISC) to yield a triplet
3
state localized on the naphthalimide moiety (D- A). In a slightly more polar solvent like di-n-butyl ether, an
3
3
+
-
equilibrium between D- A and (D -A ) is observed by means of transient absorption spectroscopy. Both species
+
-
decay with an overall decay time of ca. 1 µs. Thus, upon changing the spin multiplicity of the D -A state from
singlet to triplet, an increase of its lifetime by three orders of magnitude is observed. In more polar solvents like
3
+
-
dioxane, THF, and acetonitrile the (D -A ) state is the only species observed in the transient absorption spectrum,
3
3
+
-
with decay times of ca. 1, 0.5, and 0.1 µs, respectively. The D- A state is the precursor state for the (D -A )
state in these solvents. It is proposed that, upon increasing solvent polarity, the singlet charge-separation process is
1
3
retarded as a result of the large driving force (-∆GS° > 1 eV), which allows the triplet pathway (D- A f D- A
f (D -A )) to compete effectively. Compound 3b possesses a somewhat weaker donor chromophore than 3a
resulting in a smaller driving force. The decay of the locally excited singlet state of 3b occurs mainly Via charge
separation in the singlet manifold (D- A f (D -A )). Only in the very polar solvent acetonitrile does the triplet
pathway become competitive, and evidence is found for the formation of (D -A ).
3
+
-
1
1
+
-
3
+
-
Introduction
transfer leads to a charge separation between the terminal
chromophores. This, however, did not always lead to a very
8
Photoinduced electron transfer processes in artificial bridged
significant increase in the lifetime of the charge separation,
probably because the energy gap between the fully charge-
separated state and the ground state in such systems tends to
be small, which implies that charge recombination occurs under
close-to-“optimal” conditions.
electron-donor (D)-electron-acceptor (A) systems are widely
studied in order to understand factors that affect charge transfer
CT) rates and efficiencies.1 One of the central goals in such
-5
(
research has been to mimic natural photosynthetic systems where
the photoexcitation process is followed by multiple electron
transfer steps, which lead to a long-lived trans-membrane
charge-separated state in high yield.6,7 Indeed, quite a number
In principle the rate of charge recombination should also be
retarded if the ground and charge-separated states of a donor-
bridge-acceptor system have a different spin multiplicity. An
illustration of this phenomenon was given as early as 1988 by
8-12
13,14
of multichromophoric systems, triads,
tetrads,
and even
15
pentads have recently been realized in which multistep charge
16
Smit and Warman in their investigation of the polymethylene-
bridged compounds 1a-d, synthesized in our laboratory, that
contain a carbazole donor and a tetrachlorophthalimide acceptor
†
Laboratory of Organic Chemistry.
‡
Laboratory of Inorganic Chemistry.
X
Abstract published in AdVance ACS Abstracts, August 1, 1996.
1
7
(
see Figure 1). While earlier investigations had shown that
(
(
1) Wasielewski, M. R. Chem. ReV. 1992, 92, 435.
1
+
-
2) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26,
the singlet charge-separated state ( (D -A )) of these com-
pounds is short lived (ca. 20 ns), Smit and Warman found that
1
98.
3) Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.;
Cotsaris, E.; Verhoeven, J. W.; Hush, H. S. J. Am. Chem. Soc. 1987, 109,
(
3
+
-
a triplet charge-separated state ( (D -A )) is populated which
has a lifetime in the microsecond range. Whether this triplet
3
258.
(4) Maruyama, K.; Osuka, A.; Mataga, N. Pure Appl. Chem. 1994, 66,
8
67.
(11) Harriman, A.; Odobel, F.; Sauvage, J.-P. J. Am. Chem. Soc. 1994,
116, 5481.
(5) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret,
C.; Balzani, V.; Barigelletti, F.; Decola, L.; Flamigni, L. Chem. ReV. 1994,
(12) Osuka, A.; Yamada, H.; Shinoda, S.; Nozaki, K.; Ohno, T. Chem.
Phys. Lett. 1995, 238, 37.
9
4, 993.
(
6) Deisenhofer, J.; Michel, H. Angew. Chem., Int. Ed. Engl. 1989, 28,
(13) Osuka, A.; Yamada, H.; Maruyama, K.; Ohno, T.; Nozaki, K.;
Okada, T.; Tanaka, Y.; Mataga, N. Chem. Lett. 1995, 591.
(14) Lee, S. J.; Degraziano, J. M.; Macpherson, A. N.; Shin, E. J.;
Kerrigan, P. K.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D. Chem.
Phys. 1993, 176, 321.
(15) Gust, D.; Moore, T. A.; Moore, A. L.; Macpherson, A. N.; Lopez,
A.; Degraziano, J. M.; Gouni, I.; Bittersmann, E.; Seely, G. R.; Gao, F.;
Nieman, R. A.; Ma, X. C. C.; Demanche, L. J.; Hung, S. C.; Luttrull, D.
K.; Lee, S. J.; Kerrigan, P. K. J. Am. Chem. Soc. 1993, 115, 11141.
(16) Smit, K. J.; Warman, J. M. J. Luminescence 1988, 42, 149.
(17) Borkent, J. H. Ph.D. Thesis, University of Amsterdam, 1976.
8
29.
(7) Huber, R. Angew. Chem., Int. Ed. Engl. 1989, 28, 848.
8) van Dijk, S. I.; Wiering, P. G.; van Staveren, R.; van Ramesdonk,
(
H. J.; Brouwer, A. M.; Verhoeven, J. W. Chem. Phys. Lett. 1993, 214,
02.
9) van Dijk, S. I.; Groen, C. P.; Wiering, P. G.; Brouwer, A. M.;
5
(
Verhoeven, J. W.; Schuddeboom, W.; Warman, J. M. J. Chem. Soc.,
Faraday Trans. 1995, 91, 2107.
(10) Roest, M. R.; Lawson, J. M.; Paddon-Row, M. N.; Verhoeven, J.
W. Chem. Phys. Lett. 1994, 230, 536.
S0002-7863(96)00980-8 CCC: $12.00 © 1996 American Chemical Society

8426 J. Am. Chem. Soc., Vol. 118, No. 35, 1996
Van Dijk et al.
Figure 1. Compounds studied by Smit and Warman (1a-d), by Anglos et al. (2), and in this work: bichromophoric (D-A) systems 3a and 3b
and single chromophore compounds 4 (A), 5a (D), and 5b (D).
3
CT state has the singlet CT state or a local triplet state ( LE) as
its precursor could not be decided unequivocally. Moreover,
the flexible nature of the bridge does not allow decisive
information about the conformation of the CT state to be
obtained.
triplet pathway was limited in these cases due to substantial
charge separation occurring in the singlet manifold.
32
In a recent study, however, Anglos et al. described the very
interesting rigidly bridged system 2 (Figure 1), which shows a
different behavior. It was found that in 2 photoexcitation of
the acceptor chromophore is followed by extremely rapid ISC
Earlier studies on intramolecular charge separation in the
triplet manifold are scarce. Studies on intermolecular triplet
charge-separation processes are, however, quite ubiquitous.
In this case the time required for ISC in the locally excited
singlet state ( LE f LE) is provided by the time it takes for
the primarily excited species to diffuse toward its partner to a
distance small enough for electron transfer to occur. Charge
separation in the triplet manifold ( LE f (D -A )) thereby
becomes an efficient process. A similar order of events can be
envisaged in intramolecular systems where D and A are linked
by long flexible bridges as in 1d or the systems studied by
Shafirovich et al.24
1
3
( LE f LE) and consecutive electron abstraction from the
powerful diaminobenzene donor across the cyclic dipeptide
bridge. Population of a triplet CT state of 2 occurred with nearly
unit quantum yield, and a lifetime of 3.35 µs (in THF) was
reported for this species, where “hole” and “electron” are
separated by seven σ bonds.
18-23
1
3
3
3
+
-
In this paper we report that a long-lived triplet CT state can
also be formed in high yield with a much smaller separation
between D and A than in 2. This has been achieved Via the
piperidine bridging scheme in the bichromophoric system 3,
where an aniline-type donor and a naphthalimide acceptor are
separated by only four σ bonds (Figure 1). The piperidine
bridge provides a conformationally well-defined system.3
As models for the isolated chromophores, N-cyclohexyl-1,8-
naphthalimide (4), N-(4-methoxyphenyl)piperidine (5a), and
N-phenylpiperidine (5b) were also studied.
3
+
-
A potential problem in generating the (D -A ) state, along
this route, in more rigidly linked intramolecular systems, is the
competition between the local ISC ( LE f LE) and singlet
3-35
1
3
1
1
+
-
charge transfer ( LE f (D -A )). Obviously the energy of
3
+
-
3
00
(
D -A ) must be lower than that of LE (ET ), which in turn
1
00
is lower (often considerably) than that of LE (ES ). Thus,
there is a large driving force for charge separation from the
Experimental Section
1
LE state, which, across short spacers, can allow extremely rapid
Materials. Spectrograde solvents were used for all fluorescence
and transient absorption measurements. Dry THF was distilled from
sodium/benzophenone prior to use. Acetonitrile was dried and stored
on neutral alumina prior to the cyclic voltammetry measurement.
Butyronitrile (Fluka) was distilled (under N atmosphere) from CaH
2 2
prior to use. 1,8-Naphthalimide was obtained from Aldrich and used
as received.
11
12 -1 3,25-27
electron transfer with rates of up to 10 -10 s .
Only
1
3
in rare cases can ISC ( LE f LE) compete with such rates.
Although intramolecular charge separation in a LE state has
3
been reported in a number of papers,2
8-31
the efficiency of the
(
18) Kapinus, E. I.; Dilung, I. I. Russ. Chem. ReV. 1988, 57, 620
translated from Ups. Khim. 1988, 57, 1087).
19) Kapinus, E. I.; Aleksankina, M. M. Russ. J. Phys. Chem. 1990, 64,
413 (translated from Zh. Fiz. Khim. 1990, 64, 2625).
20) Becker, H. G. O.; Lehnmann, T.; Zieba, J. J. Prakt. Chem. 1989,
31, 806.
(
Instrumentation and Procedures. All measurements were made
(
at room temperature. Infrared (IR) spectra were obtained from CHCl
solutions, using a Perkin-Elmer 298 spectrometer. Proton nuclear
3
1
3
(
1
3
magnetic resonance ( H NMR) spectra were recorded in CDCl using
(
(
21) Whitten, D. G. ReV. Chem. Intermed. 1978, 2, 107.
22) Connolly, J. S.; Hurley, J. K.; Bell, W. L. In NATO ASI Series C214;
(28) Schmidt, J. A.; McIntosh, A. R.; Weedon, A. C.; Bolton, J. R.;
Connolly, J. S.; Hurley, J. K.; Wasielewski, M. R. J. Am. Chem. Soc. 1988,
110, 1733.
(29) Delaney, J. K.; Mauzerall, D. C.; Lindsey, J. S. J. Am. Chem. Soc.
1990, 112, 957.
(30) Hirota, J.; Okura, I. J. Phys. Chem. 1993, 97, 6867.
(31) Fraser, D. D.; Bolton, J. R. J. Phys. Chem. 1994, 98, 1626.
(32) Anglos, D.; Bindra, V.; Kuki, A. J. Chem. Soc., Chem. Commun.
1994, 2, 213.
Balzani, V., Ed.; D. Reidel: Dordrecht, The Netherlands, 1987.
23) Yasuike, M.; Shima, M.; Koseki, K.; Yamaoka, T.; Sakuragi, M.;
Ichimura, K. J. Photochem. Photobiol. A 1992, 64, 115.
24) Shafirovich, V. Y.; Batova, E. E.; Levin, P. P. Z. Phys. Chem. 1993,
82, 254.
25) Macpherson, A. N.; Liddell, P. A.; Lin, S.; Noss, L.; Seely, G. R.;
(
(
1
(
DeGraziano, G. R.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem.
Soc. 1995, 117, 7202.
(
26) Pasman, P.; Mes, G. F.; Koper, N. W.; Verhoeven, J. W. J. Am.
Chem. Soc. 1985, 107, 5839.
27) Johnson, D. G.; Niemcyzk, M. P.; Minsek, D. W.; Wiederrecht, G.
P.; Svec, W. A.; Gaines, G. L., III; Wasielewski, M. R. J. Am. Chem. Soc.
993, 115, 5692.
(33) Mes, G. Ph.D. Thesis, University of Amsterdam, 1985.
(34) Wegewijs, B.; Ng, A. F. K.; Rettschnick, R. P. H.; Verhoeven, J.
W. Chem. Phys. Lett. 1992, 200, 357.
(35) Verhoeven, J. W.; Scherer, T.; Willemse, R. J. Pure Appl. Chem.
1993, 65, 1717.
(
1

NoVel Piperidine-Bridged Donor-Acceptor Systems
J. Am. Chem. Soc., Vol. 118, No. 35, 1996 8427
a Bruker AC 200 (200 MHz), Bruker WM 250 (250 MHz), or a Bruker
ARX 400 (400 MHz) spectrometer. The latter was also used for 13
C
3
NMR (APT) and COSY spectra in CDCl . Chemical shifts are given
in ppm downfield from tetramethylsilane. Melting points are not
corrected. Column chromatography was performed with the indicated
solvent using Janssen Chimica silicagel (0.030-0.075 mm grain). R
f
values were obtained by using thin-layer chromatography (TLC) on
silicagel-coated plastic sheets (Merck silicagel 60 F254) with the same
solvent as for column chromatography. High-resolution electron impact
mass measurements (MS) were carried out using a Jeol JMS-SX/
SX102A tandem mass spectrometer.
Figure 2. Numbering scheme of 3a; H and C atoms in 3b and 4 are
labeled correspondingly.
resonances is shown in Figure 2. Wherever appropriate, this system
of numbering is maintained throughout the experimental section.
The electronic absorption spectra were recorded on a Hewlett Packard
1
13
Assignment of the H and C NMR signals was done on the basis of
8
451A diode array spectrophotometer. Fluorescence spectra were
1
1
13
1
H- H shift-correlated 2D NMR (COSY), C- H COSY, and long-
measured using a Spex Fluorolog 2 with correction for the wavelength
dependence of the detection system containing a RCA C31034
photomultiplier. Fluorescence decay curves of 3a and 4 were measured
by means of time-correlated single photon counting (SPC) (λex ) 317
range 13C- H COSY.
1
N-(1-(4-Methoxyphenyl)-4-piperidinyl)-1,8-naphthalimide (2-[1-
(4-methoxyphenyl)piperidin-4-yl]benz[de]isoquinoline-1,3-dione) (3a).
nm, fwhm ) ca. 22 ps). The setup has been described earlier.9
The reaction was carried out under a dry-N atmosphere. 1,8-Naphthalic
2
A
anhydride (1.64 g, 8.24 mmol) was dissolved in 200 mL of DMF and
heated to 120 °C. A solution of 1-(4-methoxyphenyl)piperidin-4-
ylamine in 50 mL of DMF was added dropwise and the mixture was
stirred over night at 120 °C, whereupon the solvent was evaporated.
total of 2048 channels of the Multi Channel Buffer (EG&G Ortec 918
ADCAM) working in a pulse-height analysis mode were used. The
channel widths used were 2.5, 5, and 10 ps/channel. Fluorescence decay
curves of 3b were measured with a nanosecond time scale setup,
3
6
The solid was dissolved in CH
filtered off. The filtrate was evaporated to dryness and the residue
was submitted to repeated crystallization from CH Cl . Crude product
was obtained in a 1.58 g yield. Column chromatography of 148 mg
2 2
Cl and 1,8-naphthalic anhydride was
described earlier, using a Lumonics Pulse Master EX748 XeCl
Excimer laser (λex ) 308 nm, fwhm ca. 7 ns) as an excitation source.
This laser was also used for recording the flash photolysis transient
absorption spectra, along with a 450-W high-pressure Xe arc as the
probe light, pulsed with a M u¨ ller Elektronik MSP05 pulser. The overall
2
2
of this solid using CH
mg, 0.25 mmol, 32%). R
2940, 2920, 2800, 1690 (CdO), 1650 (CdO), 1615, 1580 (CdC). H
2
Cl
2
as eluent afforded 3a as a yellow solid (90.8
-1
9
f
0.20. Mp 212.7-214.1 °C. IR (cm ) 2980,
time resolution of this setup is ca. 10 ns. Concentrations of the studied
1
-
5
compounds were ca. 10 M for the fluorescence measurements and
-
4
NMR (400 MHz) δ 8.58 (d, 2H; H23, H29), 8.19 (d, 2H; H25, H27),
ca. 10 M for transient absorption measurements (A ) 0.1 at λex in a
-cm cell). The samples were carefully deoxygenated by purging with
7
2
.74 (t, 2H; H24, H28), 6.98-6.94 (m, 2H; H8, H12), 6.86-6.82 (m,
H; H9, H11), 5.14 (m, 1H; H4), 3.77 (s, 3H, OCH ), 3.66 (d, 2H;
1
3
argon (ca. 15 min) or by repetitive freeze-pump-thaw cycles.
Spectroelectrochemistry. The UV-vis spectroelectrochemical
experiment with 4 was performed on a Perkin-Elmer Lambda 5 UV-
vis spectrophotometer connected to a 3600 data station. An OTTLE
H2eq, H6eq), 3.01 (qd, 2H; H3eq, H5eq), 2.84 (t, 2H; H2ax, H6ax), 1.78
13
(
(
d, 2H; H3ax, H5ax). C NMR (100 MHz) 164.60 (C16, C20), 153.66
C10), 146.14 (C7), 133.63 (C25, C27), 131.47 (C26), 131.19 (C23,
3
7
C29), 128.20 (C18), 126.95 (C24, C28), 123.13 (C17, C19), 118.96
C8, C12), 114.37 (C9, C11), 55.58 (C14), 51.95 (C4), 51.77 (C2, C6),
8.44 (C3, C5). High-resolution MS: found m/z 386.1630; calcd for
m/z 386.1630.
N-Cyclohexyl-1,8-naphthalimide (2-cyclohexylbenz[de]isoquino-
cell equipped with a Pt-minigrid working electrode (32 wires/cm)
and quartz/CaF windows was used at room temperature. The working
(
2
2
electrode surroundings were masked carefully to avoid spectral interfer-
ence with the non-electrolyzed solution. Controlled-potential elec-
trolysis within the OTTLE cell was carried out by a PA4 (EKOM,
Czech Republic) potentiostat. The sample solution in butyronitrile was
24 22 2 3
C H N O
line-1,3-dione) (4). A solution of cyclohexylamine (1.5 g, 15 mmol)
and acetic acid (3 mL) in 45 mL of DMF was added dropwise to 1,8-
naphthalic anhydride (2.0 g, 10 mmol) in 30 mL of DMF at 40 °C.
The mixture was stirred at 40 °C for 30 min and at 150 °C for 4 h.
After the mixture was cooled to room temperature the solvent was
evaporated and the residue was washed with water and dissolved in
prepared and handled under an N
2
atmosphere. The concentrations of
NPF supporting electrolyte
were 5 × 10 and 3 × 10 M, respectively.
the electrolyzed compound 4 and the (nBu)
4
6
-
3
-1
The reduction potential of 4 was determined by cyclic voltammetry
using a gas-tight three-electrode cell and a Bank Electronic POS 73
Wenking Potentioscan potentiostat coupled to a HP 7090A measurement
plotting system. The measurements were carried out in deoxygenated
acetonitrile containing tetraethylammonium tetrafluoroborate (TEAFB)
CH
2
Cl
2
. The CH
2
Cl
2
layer was washed with water and saturated sodium
SO , and the solvent was
bicarbonate solution and dried with Na
2
4
evaporated. The residue was dissolved in DMF/acetic acid, a saturated
sodium bicarbonate solution was added, and the mixture was heated
for 30 min at ca. 100 °C. The solid was filtered off and dissolved in
(ca. 0.1 M) as supporting electrolyte and at sweep rates of 100-400
mV/s. A platinum disk (2 mm) working electrode with a Pt gauze
auxiliary electrode was used in combination with a saturated calomel
reference electrode (SCE) connected to the cell Via a 3M KCl salt
bridge.
CH
2
Cl
2
. This solution was washed with water and saturated NaHCO
3
,
dried with Na
2
SO , and evaporated to dryness. Crystallization of the
4
residue from ethanol yielded 4 as light yellow needles (1.22 g, 4.37
mmol, 44%). Mp 231-233 °C. IR 3080-3000, 2930, 2850, 1695,
Synthesis. The method used for the synthesis of the imide systems
was reported earlier by Demmig and Langhals.38 Reaction of 1-(4-
methoxyphenyl)piperidin-4-ylamine or 1-phenylpiperidin-4-ylamine
with 1,8-naphthalic anhydride affords the bichromophoric systems 3a
and 3b, respectively. The acceptor-model system 4 is obtained from
reaction of cyclohexylamine with 1,8-naphthalic anhydride. 1-Phen-
1
1
8
2
655, 1630, 1590. H NMR (250 MHz) δ 8.52 (d, 2H; H23, H29),
.14 (d, 2H; H25, H27), 7.70 (t, 2H; H24, H28), 5.01 (“t”, 1H; H4),
.54 (q, 2H; H3ax, H5ax), 1.89 (“d”, 2H; H3eq, H5eq), 1.73 (“d”, 3H;
H1eq, H2eq, H6eq), 1.55-1.22 (m, 3H; H1ax, H2ax, H6ax). High-resolution
MS: found m/z 279.1254; calcd for C18 17NO m/z 279.1259. UV
H
2
-1
-1
-1
-1
(
acetonitrile), λ(ꢀ): 332 (12300 M cm ), 346 (11300 M cm ).
3
9
ylpiperidin-4-ylamine was synthesized Via a known route from
N-(1-Phenyl-4-piperidinyl)-1,8-naphthalimide (2-(1-phenylpip-
eridin-4-yl)benz[de]isoquinoline-1,3-dione) (3b). The same method
was employed as described for 4 using 1-phenylpiperidin-4-ylamine
1
-phenylpiperidone (Via the oxim derivative). 1-(4-Methoxyphenyl)-
piperidin-4-ylamine was obtained analogously from 1-(4-methoxyphen-
yl)piperidone. The synthesis of the donor-reference systems 5a and
(
(
2.05 g, 12 mmol), acetic acid (2 mL), and 1,8-naphthalic anhydride
1.34 g, 7 mmol). Crystallization from CH Cl /n-hexane afforded 3b
40
5
b has been described elsewhere. Numbering for assignment of NMR
2
2
as yellow needles (250 mg, 0.70 mmol, 10%). Mp 249-250 °C. IR
(
(
36) Ramesdonk, H. J.; Verhoeven, J. W. Tek Imager 1989, 1, 2.
37) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. Interfacial
1
3
100-2860, 2810, 1695, 1655, 1625, 1590. H NMR (200 MHz) δ
8
.57 (d, 2H; H23, H29), 8.18 (d, 2H; H25, H27), 7.74 (t, 2H; H24,
Electrochem. 1991, 317, 179.
(
38) Demmig, S.; Langhals, H. Chem.Ber. 1988, 121, 225.
H28), 7.37-7.18 (m, 2H; H9, H11), 6.99 (d, 2H; H8, H12), 6.83 (t,
1H; H10), 5.23 (“t”, 1H; H4), 3.84 (“d”, 2H; H2eq, H6eq), 3.20-2.80
(39) Scherer, T.; Hielkema, W.; Krijnen, B.; Hermant, R. M.; Eijckelhoff,
C.; Kerkhof, F.; Ng, A. K. F.; Verleg, R.; van der Tol, E. B.; Brouwer, A.
M.; Verhoeven, J. W. Recl. TraV. Chim. Pays-Bas 1993, 112, 535.
(40) Krijnen, L. B. Ph.D. Thesis, University of Amsterdam, 1990.

8428 J. Am. Chem. Soc., Vol. 118, No. 35, 1996
Van Dijk et al.
Table 1. Half-Wave Potentials of the Oxidation (Eox) and
Reduction (Ered) of Separate Chromophore Models (see Figure 1)
(
Cyclic Voltammetry at the Pt Disk Electrode in Acetonitrile Vs
SCE), along with Some Photophysical Parameters (in Cyclohexane)
redox
potentials (V)
E00 (eV)
0
0
00
chromophore
E
S
E
T
E
ox
E
red
a
a
c
-1.34g
N-cyclohexyl-1,8-naphthalimide (4) 3.63 2.29
N-(4-methoxyphenyl)piperidine (5a) 3.69 2.99 0.61d
b
e
N-phenylpiperidine (5b)
3.91 2.95f 0.81d
a
00
Data for N-methyl-1,8-naphthalimide, ref 41; E
T
from phosphor-
b
escence at 77 K. Data for N,N-dimethyl-4-methoxyaniline, ref 52.
Data for N,N-dimethyl-4-methoxyaniline, ref 64. Reference 52.
Reference 65. Data for N,N-diethylaniline, ref 66. This work.
c
e
d
f
g
Figure 3. UV-vis spectra of the radical cation of 1-(4-methoxyphen-
yl)-4-piperidine in acetonitrile (dashed line), scaled to ꢀ ) 6000 at 505
(
m, 4H; H2ax, H6ax, H3eq, H5eq), 1.80 (“d”, 2H; H3ax, H5ax). High-
44
nm, and the radical anion of 4 in butyronitrile (solid line).
resolution MS: found m/z 356.1523; calcd for C23
3
M
H
20
N
2
O
2
m/z
-1
-1
56.1525. UV (acetonitrile), λ(ꢀ): 256 (14800 M cm ), 332 (13500
M- cm ), and 818 nm (ꢀ ) 5100 M cm ). (The extinction
coefficients given have been corrected for the 95% conversion
of 4 into its radical anion, which could maximally be achieved
in the spectroelectrochemical reduction under retention of
isosbestic points.)
1
-1
-1
-1
-
1
-1
-1
-1
cm ), 346 (12300 M cm ).
Results
Separate Chromophores. Compound 4 was used as a
reference system for the acceptor chromophore whereas 5a and
b served as donor references. Relevant photophysical and
Bichromophoric System 3a. Compound 3a displays in
cyclohexane a weak local emission in the 370-nm region (τ )
5
redox data are compiled in Table 1. Clearly, the lowest locally
excited singlet state of 3 is located on the imide acceptor moiety
(
chromophore also absorbs most of the light (ꢀ308(3a) ) 8500 L
mol cm , ꢀ308(3b) ) 7000 L mol cm , ꢀ308(4) ) 6000 L
mol cm ), and very rapid energy transfer D-A f D- A
is likely to occur anyway in the smaller fraction of molecules
which are initially excited in the donor unit. It is known that
naphthalimides undergo efficient intersystem crossing causing
a short singlet-state lifetime and low fluorescence quantum
0
.3 ns). In the 600-nm region, a very weak, broad CT emission
-
4
was observed (λmax ) ca. 610 nm, φf < 4 × 10 ), which grew
with a time constant of 0.25 ns and decayed with τf ) 0.75 ns.
From the decrease of the fluorescence decay time of the locally
excited state emission relative to compound 4, we can estimate
that the charge-separation process for 3a in cyclohexane, which
affords the singlet CT state (D- A f (D -A )), occurs with
ca. 70% yield and a rate of ca. 4 × 10 s .
00
ES (A) ) 3.63 eV, Table 1). Upon excitation at 308 nm this
-
1
-1
-1
-1
-
1
-1
1
1
1
1
+
-
9
-1
In a polar medium like acetonitrile, no CT emission was found
-
3
for 3a. The local emission (λmax ) 372 nm, φf ) 1 × 10 ) in
this solvent is stronger than that in cyclohexane. In fact, the
intensity is equal, within the limits of accuracy ((10%), to that
of 4 in acetonitrile, indicating that for the decay of the locally
excited singlet state of 3a in acetonitrile charge transfer is a
minor route.
1
7,41,42
yields.
Indeed, compound 4 displays only very weak
-4
emission (φf ) 2 × 10 (τf ) 1.0 ns) in cyclohexane, increasing
-
3
to φf ) 1 × 10 in acetonitrile), with maxima at 376 and 380
nm. The intersystem-crossing yield of N-methyl-1,8-naphthal-
imide has been reported to decrease slightly from 1.0 in hexane
to 0.94 in acetonitrile.42 It seems reasonable to assume that
the intersystem-crossing yields of 4 are also close to unity.
In nanosecond flash photolysis, the excited state absorption
spectrum of 4 in cyclohexane (Figure 4a) shows a structured
absorption band with maxima at 352, 412 (sh), 436, and 465
nm that can be attributed to the triplet state of the acceptor
The transient absorption spectra of 3a in cyclohexane reveal
clearly the local triplet state of the acceptor with absorption
maxima at 355, 412 (sh), 437, and 464 nm, which decay on a
microsecond time scale (Figure 4A). At t ) 0 ns (maximum
of the laser pulse) a short-lived (<2 ns) band at 414 nm is,
however, present which can reasonably be ascribed to the radical
anion absorption of the imide acceptor (see Figure 3). Thus,
4
2
43
chromophore with an apparent decay time of τ ) 14.4 µs.
To allow accurate assignment of the transient absorption
spectra of the bichromophoric systems, the UV-vis spectra of
the radical ions of isolated chromophores were obtained. The
1
+
-
the short-lived (D -A ) state that was observed by its long-
wavelength emission could also be identified in the transient
absorption spectrum. We note that spectra were recorded at
intermediate points in time which are not shown in Figures 4
and 5.
UV-Vis spectra of the radical cations of N-phenylpiperidine
(
yl)piperidine (λmax 505 nm (ꢀ ) 6000 M cm )) are known
from the literature. The spectra of the radical cation of N-(4-
methoxyphenyl)piperidine and of the radical anion of 4, the latter
recorded by application of UV-vis spectroelectrochemistry, are
shown in Figure 3. The absorption maxima of the radical anion
of 4 in butyronitrile are observed at 229 (ꢀ ) 21200 M cm ),
2
λmax 485 nm (ꢀ ) 4600 M-1 cm )) and of N-(4-methoxyphen-
-1
-
1
-1
In the transient absorption spectra of 3a in di-n-butyl ether
4
4
(
(
Figure 4B) the acceptor triplet is visible (352, 466 nm)
compare with Figure 4A), as well as bands corresponding with
+
-
the D and A radical ion absorptions (see Figure 3). All main
-
+
-
3
absorptions (414 nm (A ) and 466 nm (D , A , and A)) decay
with the same time constant of ca. 1 µs. In the long-wavelength
region of the absorption spectrum, a weak absorption band can
be discriminated at ca. 800 nm. The noise on this band, which
is a result of the low probe light intensity and low sensitivity
of the detector in that wavelength region, makes assignment
dubious. We note, however, that the radical anion of the
acceptor reference system 4 displays an absorption in this long-
wavelength region (see Figure 3).
-1
-1
-
1
-1
-1
-1
69 (ꢀ ) 16600 M cm ), 347 (ꢀ ) 3800 M cm ), 416 (ꢀ
-1
-1
-1
-1
)
23550 M cm ), 489 (ꢀ ) 3550 M cm ), 736 (ꢀ ) 3000
(
41) Korol’kova, N. V.; Val’kova, G. A.; Shigorin, D. N.; Shigalevskii,
V. A.; Vostrova, V. N. Russ. J. Phys. Chem. 1990, 84, 206.
42) Wintgens, V.; Valat, P.; Kossanyi, J.; Bicz o´ k, L.; Demeter, A.;
B e´ rces, T. J. Chem. Soc., Faraday Trans. 1994, 90, 411.
43) A single exponential function was used to describe the decay,
although it is known that the triplet decay in naphthalimides involves
mixed first- and second-order kinetics.
(
(
4
2
In benzene (not shown), the absorption spectra are dominated
(44) Brouwer, A. M.; Mout, R. D.; Maassen van den Brink, P. H.;
3
+
-
Warman, J. M.; Jonker, S. A. Chem. Phys. Lett. 1991, 180, 556.
by long-lived (τ ) ca. 0.9 µs) absorptions of (D -A ), with

NoVel Piperidine-Bridged Donor-Acceptor Systems
J. Am. Chem. Soc., Vol. 118, No. 35, 1996 8429
Figure 4. Transient absorption spectra of (A) 4 (dotted line) and 3a
in cyclohexane (t ) 0 ns (solid line); t ) 500 ns (dashed line)), (B) 3a
in di-n-butyl ether, and (C) 3a in THF. The intensity of the spectrum
of 4 corresponds to φISC ) 1 (see text). The intensity of the spectrum
of 3a in cyclohexane at t ) 0 ns corresponds to φISC ) 0.6 (see text).
maxima at 416 and 500 nm, that correspond with the absorption
-
+
spectra of the A and D radical ions. At t ) 0 ns (maximum
of the laser pulse) and at t ) 5 ns, a minor absorption due to
3
D- A is observed at 466 nm.
In dioxane and the more polar solvents THF and acetonitrile
only two long-lived absorption bands are observed at ca. 415
nm and ca. 491 nm, which correspond with the acceptor and
donor radical ions, respectively. This is illustrated for the THF
case in Figure 4C. Note that, again, weak absorption is present
in the >750-nm region that can be attributed to the radical anion.
The decay time of the radical ion absorptions in dioxane is ca.
Figure 5. Transient absorption spectra of 3b in (A) cyclohexane, (B)
di-n-butyl ether, (C) THF, and (D) acetonitrile.
1
µs (i.e. the same as in di-n-butyl ether). In THF and
(
τ < 1 ns) and >660 nm, respectively. This CT emission
3
+
-
acetonitrile the energy gap between the (D -A ) state and
the ground state is reduced, which results in a decrease of the
lifetime to 0.5 and 0.1 µs, respectively (see Discussion).
Bichromophoric System 3b. To obtain further information
on the relation between the energy of the CT state and the
formation of a triplet CT state, we studied the system with a
somewhat weaker donor (3b) (Table 1). In cyclohexane, di-
n-butyl ether, and diethyl ether weak, short-lived CT emission
bands are observed with maxima at ca. 570 (τ < 1 ns), ca. 630
demonstrates the occurrence of a singlet state charge-separation
process in 3b in these solvents.
To obtain further information on the excited state decay
pathways, transient absorption experiments were performed
(Figure 5). In cyclohexane, the only state observed for 3b is
the imide triplet state with maxima at 354, 439 and 467 nm
(Figure 5a) and an apparent decay time of 29.9 µs. The (D -
A ) state, which was detected by its fluorescence, could not be
1
+
-

8430 J. Am. Chem. Soc., Vol. 118, No. 35, 1996
Van Dijk et al.
observed with our nanosecond transient absorption apparatus
3
+
-
nor was a long-lived (D -A ) state found.
Also in di-n-butyl ether the triplet absorptions, originating
from the imide chromophore, dominate the transient spectra of
3b (λmax ) 355, 440, 467 nm, Figure 5B). At 416 nm, however,
a short-lived transient absorption band is observed, which
corresponds with the main absorption of the radical anion of
the acceptor. Thus, the 1(D -A ) state, detected by CT
fluorescence, is now also detected by transient absorption. In
THF, transient absorptions that correspond with the radical ions
+
-
-
+
-
are observed at 418 nm (A ) and 469 nm (D , A ) (Figure
C). These absorptions decay on a time scale of ca. 10 ns.
5
Virtually no imide triplet state is observed in this solvent. This
result implies that in THF the singlet charge-separated state is
formed efficiently upon photoexcitation, but this does not lead
Figure 6. Energy scheme for the excited states of 3a in cyclohexane,
di-n-butyl ether, and THF (see text).
3
+
-
to a (D -A ) state.
by ca. three orders of magnitude. The radical ion absorption
In acetonitrile, absorptions at 360, 414, and 468 nm are
+
-
bands only indicate the presence of D -A , but do not give
+
-
3
observed that can again be attributed to D , A , and A (Figure
D). The decay time of these species is in the microsecond
range, indicating that they result from an equilibrium between
information about the spin state. The very long lifetime,
5
+
-
however, can only be explained assuming that the D -A state
has triplet multiplicity.
3
3
+
-
D- A and (D -A ).
An estimate of the energies of the charge-separated states
4
5
can be obtained by applying eq 1, in which the energy of the
Discussion
+
charge-separated state ED A- (in eV) is calculated by consider-
ing it as a solvent separated ion pair with the center-to-center
distance R (in Å) and effective ionic radii r (in Å) equal for D
and A, submerged in a dielectric continuum with relative
permittivity ꢀs. Even though the bridges in linked systems
occupy part of the space available to the solvent in nonbridged
ion pairs, it has been found that eq 1 can be applied successfully,
at least if the bridges are arranged in an extended fashion as in
From the fluorescence and transient absorption data it was
concluded that upon photoexcitation of 3a/3b the decay
1
pathways of the locally excited singlet state (D- A) consist of
1
3
two competing processes Viz. ISC (D- A f D- A) and charge
1
1
+
-
separation (D- A f (D -A )). The transient absorption band
at 415 nm, corresponding with A , and the band at 465 nm,
which corresponds with D- A, are clear marker bands that
-
3
3
,9,46-50
3
a and 3b.
provide a powerful tool for the assignment of the observed
transient absorptions.
E
D A
+ -
) E (D) - E (A) + (14.4/r)(1/ꢀ - 1/37.5) -
ox
red
s
For 3a in cyclohexane charge separation occurs in the singlet
manifold with ca. 70% yield. The CT fluorescence grew with
the same rate constant as the local emission decayed (0.3 ns)
indicating that, as expected, the locally excited singlet acceptor
state is the precursor state of the (singlet) charge-separated state.
The short-lived singlet CT state is observed not only by its
emission but also in the transient absorption spectra. The
nanosecond transient absorption spectra of 3a in cyclohexane
1
4.4/(ꢀ R) (1)
s
For the D and A species under study, the one-electron oxidation
and reduction potentials (Eox(D) and Ered(A)) in acetonitrile are
known (see Table 1). To obtain an estimate of the CT-state
energies in different solvents, we set the ionic radii r at 4.1 Å,
which experience has shown to be a reasonable value for small
5
1,52
aromatic systems.
The distance between the centers of
(
Figure 4A), detected at the maximum of the laser pulse, show
-
charge of the donor and acceptor is denoted in eq 1 by the
symbol R. For N-(4-methoxyphenyl)piperidine the center of
charge is likely to be close to the nitrogen. For the acceptor
chromophore the center of charge should coincide with the
center of the chromophore. We thus obtained R ) 6.8 Å. These
parameters have been used to construct Figure 6. For the locally
excited states the data of Table 1 have been applied. The energy
diagram obtained for 3a in cyclohexane is in good agreement
with the excited state behavior that was deduced from fluores-
cence and transient absorption spectra. In Figure 6 it can be
a short-lived absorption that corresponds with A . The spectra
are dominated, however, by an absorption band that can be
42
assigned to a triplet state localized on the imide chromophore.
By comparison with a measurement on 1,8-naphthalimide
4
2
3
(
φISC ) 0.95), the yield of D- A for 3a in cyclohexane was
estimated to be ca. 0.6. We therefore scaled the intensity of
the absorption of 3a in Figure 4A at t ) 0 ns (maximum laser
pulse) to φISC ) 0.6 at λmax (465 nm). After 500 ns hardly any
decay of the acceptor triplet state has occurred, while the 414-
nm band has disappeared due to decay of the CT state.
Fluorescence measurements showed that the yield of the charge
separation from the locally excited singlet state of 3a in
cyclohexane is ca. 0.7. The contribution of direct ISC from
3
+
-
seen that the degeneracy of D- A and D -A states in di-n-
butyl ether, as revealed by the transient absorption spectra, is
predicted by eq 1. An equilibrium between a locally excited
1
1
3
D- A (D- A f D- A) is therefore ca. 0.3. We determined
that the total ISC yield observed is ca. 0.6, hence about half of
(45) Weller, A. Z. Phys. Chem. Neue Folge 1982, 133, 93.
(
46) Gaines, G. L.; O’Neil, M. P.; Svec, W. A.; Niemczyk, M. P.;
Wasielewski, M. R. J. Am. Chem. Soc. 1991, 113, 719.
47) Irvine, P. M.; Harrison, R. J.; Beddard, G. S.; Leighton, P.; Sanders,
3
the D- A state population is obtained by back electron transfer
with ISC from the (D -A ) state ( (D -A ) f D- A).
Upon increasing the solvent polarity, ISC (D- A f D- A)
can compete effectively with charge separation in the singlet
manifold (D- A f (D -A )). This leads to an equilibrium
for 3a in di-n-butyl ether between D- A and (D -A ), both
species decaying with an apparent decay time of ca. 1 µs (Figure
(
1
+
-
1
+
-
3
J. K. M. Chem. Phys. 1986, 104, 315.
(48) Joran, A. D.; Leland, B. A.; Felker, P. M.; Zewail, A. H.; Hopfield,
J. J.; Dervan, P. B. Nature 1987, 508.
1
3
(49) Schmidt, J. A.; Liu, J.-Y.; Bolton, J. R.; Archer, M. D.; Gadzekpo,
1
1
+
-
V. P. Y. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1027.
(50) Warman, J. M.; Smit, K. J.; de Haas, M. P.; Jonker, S. A.; Paddon-
Row, M. N.; Oliver, A. M.; Kroon, J.; Oevering, H.; Verhoeven, J. W. J.
Phys. Chem. 1991, 95, 1979.
3
3
+
-
4
B). Thus, upon changing the multiplicity of the charge-
(51) Kroon, J. Ph.D. Thesis, University of Amsterdam, 1992.
separated state from singlet to triplet, its decay time is increased
(52) Scherer, T. Ph.D. Thesis, University of Amsterdam, 1994.

NoVel Piperidine-Bridged Donor-Acceptor Systems
J. Am. Chem. Soc., Vol. 118, No. 35, 1996 8431
+
-
triplet state and a D -A state was also reported by Anglos et
al. for their bichromophoric system 2 in toluene.32
The absorption corresponding with the radical cation is over-
3
shadowed by the strong D- A absorption. The transient
absorption spectra in both cyclohexane and di-n-butyl ether are
From the transient absorption data of 3a in di-n-butyl ether
Figure 4B) it is concluded that the (D -A ) and D- A states
are similar in energy in this solvent, because D , A , and A
absorptions decay with the same time constant. In benzene,
however, the transient absorption spectra of 3a are dominated
3
3
+
-
3
dominated by the D- A absorption. This situation is thus
(
+
-
3
similar to that of 3a in cyclohexane (Figure 6).
1
+
-
Upon photoexcitation of 3b in THF, the (D -A ) state is,
again, readily formed as is observed by short-lived transient
+
-
+
-
by long-lived absorptions that correspond with D and A . Only
during the laser pulse are D- A absorptions observed. We
absorption bands that correspond with D and A . Thus in
3
1
1
+
THF charge separation in the singlet manifold (D- A f (D -
1
-
1
3
tentatively assume that for 3a in benzene, formation of D- A
A )) can still prevail over ISC (D- A f D- A). In this case
1
3
3
upon photoexcitation is followed by rapid ISC (D- A f D- A)
virtually no D- A absorption is observed, indicating that, for
3
1
+
-
to yield the D- A state, and from this state charge transfer
3b in THF, the (D -A ) state decays directly to the ground
3
+
-
occurs to afford the (D -A ) state. Thus in benzene, the
state. By applying eq 1 and the parameters denoted above, it
3
+
-
3
+
-
3
(
D -A ) state is lower in energy than the D- A state, whereas
was estimated that, for 3a in THF, the D -A and D- A states
+
-
00
in di-n-butyl ether these two states are degenerate. Hence
benzene acts as a slightly more polar solvent than di-n-butyl
ether for the charge-separated state of 3a. It is, however,
known53 that benzene stabilizes dipolar states more effectively
than expected from its bulk dielectric properties.
are virtually degenerate (E(D -A ) - ET ) 2.24 - 2.29 )
-0.05 eV), which implies that there will be a small barrier, of
the order of one fourth of the Marcus reorganisation energy,
1
+
-
3
for the (D -A ) f D- A process.
1
+
-
Importantly, the population of (D -A ) does not lead to
3
+
-
formation of (D -A ). The short distance between the radical
Upon photoexcitation of 3a in more polar solvents like
1
3
ions in our systems apparently allows for sufficient exchange
dioxane, THF, and acetonitrile, rapid ISC (D- A f D- A)
1
+
interaction to give pure singlet and triplet character to the (D -
occurs, which is followed by charge separation in the triplet
-
3
+
-
56-58
3
+
3
3
+
-
3
+
-
A ) and (D -A ) states, respectively.
The “pure” (D -
manifold (D- A f (D -A )). The (D -A ) state is clearly
-
3
+
-
A ) state is apparently populated most effectively Via the D- A
state.
indicated by absorption bands that correspond with D and A .
In fact, the transient absorption spectra are a superposition of
3
+
-
these bands. The decay time of the (D -A ) state of 3a
decreases upon increasing solvent polarity, i.e. from 1 µs to
.5 µs and 0.1 µs in dioxane, THF, and acetonitrile, respectively.
This behavior corresponds with the “energy gap-law”,
strongly suggests that the decay pathway of the (D -A ) state
is directly to the ground state, rather than by ISC within the
D -A state ( (D -A ) f (D -A )). For the latter spin-
inversion process a large solvent effect is not expected.
Figure 6 documents, according to eq 1, that the charge-
separated state is energetically accessible from the D- A state
in polar solvents like THF, which is in full agreement with the
observed behavior. The rate of ISC (D- A f D- A) in the
isolated naphthalimide chromophore (4) is somewhat smaller
in acetonitrile than in cyclohexane. The lack of charge transfer
in the singlet manifold (D- A f (D -A )) of 3a in
acetonitrile can therefore not be the consequence of faster ISC
D- A f D- A), but must be due to retardation of the singlet
charge transfer itself (see below).
We will now discuss bichromophoric system 3b. From the
experimental data of 3b it is concluded that the decay of its
locally excited singlet state occurs Via charge separation in the
singlet manifold (D- A f (D -A )). Only in the very polar
solvent acetonitrile there is evidence for the formation of a triplet
charge-separated state. For 3b, the CT emission in cyclohexane
is found at a shorter wavelength (ca. 570 nm) compared with
that of 3a in the same solvent (ca. 610 nm), as expected for a
weaker donor-acceptor pair (see eq 1, Table 1).
In acetonitrile, the transient absorption spectra of 3b display
+
-
long-lived absorptions that can be attributed to D , A , and
3
0
A. This proves that compound 3b can afford a long-lived
54,55
3
+
-
which
charge-separated (D -A ) state, albeit only in a very polar
medium.
3
+
-
3
+
-
From Figure 6 it is clear that for the (D -A ) state to
+
-
3
+
-
1
+
-
3
become energetically accessible from D- A, a more polar
solvent is necessary for 3b than for 3a. Apart from this trivial
3
+
-
1
prerequisite, formation of (D -A ) requires that ISC (D- A
3
3
f D- A) is fast relative to the charge separation in the singlet
1
1
+
-
manifold (D- A f (D -A ), which in turn implies that the
1
3
latter process has to be slowed down. The relative rate of ISC
1
3
(D- A f D- A) versus the rate of charge separation in the
1
1
+
-
42
singlet manifold (D- A f (D -A )) from the locally excited
singlet state is discussed in the next paragraphs.
1
1
+
-
The driving force for charge separation in the singlet manifold
4
5
1
3
(∆G °) can be obtained from eq 2
(
S
1
00
∆G ° ) E
D A
+ -
- ES
(2)
S
where ES00 denotes the energy of the locally excited acceptor
1
1
+
-
state (3.63 eV, Table 1). From eq 1 and using the thermody-
namic parameters given above, an estimate of the energy of
1
+
the singlet charge-separated state ( ED A-) can be obtained.
Using eq 2, ∆GS° was estimated to be -1.03 and -1.74 eV for
3
a, and -0.83 and -1.54 eV for 3b, in cyclohexane and
acetonitrile, respectively. Compound 3a possesses a very strong
donor-acceptor pair, which results in a very large driving force
already in cyclohexane, which increases substantially in more
The transient absorption spectra of 3b in cyclohexane display
a long-lived triplet absorption that is localized on the imide
moiety. The short-lived (D -A ) state, which was detected
by its emission, was not observed in the transient absorption
spectrum. In the more polar solvent di-n-butyl ether, however,
1
+
-
59
polar solvents. It has been reported by Wasielewski et al.
that for such large energy gaps (-∆G ° > 1 eV) the rate of the
S
charge separation decreases, irrespective of the solvent used,
-
the A marker band at 416 nm is observed, which decays with
as the energy gap increases (the well-known “inverted region
a time constant in the nanosecond range. Thus for 3b in di-
1
+
-
(56) Hasharoni, K.; Levanon, H.; Greenfield, S. R.; Gosztola, D. J.; Svec,
W. A.; Wasielewski, M. R. J. Am. Chem. Soc. 1995, 117, 8055.
n-butyl ether, the (D -A ) state is evidenced by both its
-
emission and the absorption band that corresponds with A .
(57) Nakamura, H.; Terazima, M.; Hirota, N.; Nakajima, S.; Osuka, A.
Bull. Chem. Soc. Jpn. 1995, 2193.
(58) Berman, A.; Izraeli, E. S.; Levanon, H.; Wang, B.; Sessler, J. L. J.
Am. Chem. Soc. 1995, 117, 8252.
(59) Wasielewski, M. R. Photoinduced Electron Transfer. Part A;
Elsevier: Amsterdam, 1988, p 173.
(
(
(
53) Suppan, P. J. Photochem. Photobiol. 1990, 50, 293.
54) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145.
55) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cum-
mings Publishing Co. Inc.: Menlo Park, CA, 1978, p 183.

8432 J. Am. Chem. Soc., Vol. 118, No. 35, 1996
Van Dijk et al.
effect”6 ). Thus for 3a, due to the large energy gap for charge
0-63
Conclusions
1
1
+
-
separation in the singlet manifold, the D- A f (D -A )
process is apparently slowed down upon increasing solvent
polarity. As a result, ISC (D- A f D- A) can compete
effectively, and the D- A state is formed in a high yield as an
The data presented above demonstrate that we were successful
in an attempt to design bichromophoric systems that afford a
1
3
3
+
-
(
D -A ) state with a lifetime in the microsecond range and a
3
charge separation by four σ bonds. The intramolecular
combination of a 4-methoxyaniline electron donor and a 1,8-
naphthalimide electron acceptor (3a) is the most successful of
the two systems studied. Compound 3a contains a very strong
donor-acceptor pair (-∆GS° > 1 eV). As a result, the singlet
3
+
-
efficient precursor for the (D -A ) state (Figure 4B,C).
For system 3b in cyclohexane, -∆GS° is estimated to be
significantly smaller than 1 eV; therefore, the rate for charge
separation in the singlet manifold is initially expected to increase
1
1
+
-
upon increasing the solvent polarity.6
0-63
charge separation (D- A f (D -A )) is retarded upon
increasing solvent polarity, and ISC (D- A f D- A) competes
effectively in benzene, di-n-butyl ether, and more polar solvents.
From the thus obtained D- A state, electron transfer occurs in
the triplet manifold, to afford a long-lived triplet charge-
separated state (D- A f (D -A )). Bichromophoric system
3b contains a somewhat weaker donor than 3a, resulting in a
smaller driving force compared with 3a in the same solvent.
Photoexcitation of 3b leads to population of the singlet charge-
separated state (D- A f (D -A )). Only in the very polar
solvent acetonitrile was evidence found for formation of a long-
lived triplet charge-separated state.
Indeed, for 3b in
1
3
nonpolar solvents and solvents of intermediate polarity, emission
1
+
-
from the (D -A ) state is observed, indicating that in these
3
1
1
+
-
solvents the D- A f (D -A ) process is fast compared with
1
3
ISC (D- A f D- A). This very rapid charge separation results
3
3
+
-
1
+
-
in formation of (D -A ) and thus prevents the triplet pathway
D- A f D- A f (D -A )) to compete. In the very polar
solvent acetonitrile, the energy gap for charge separation in the
singlet manifold in 3b is significantly larger than 1 eV (-∆GS°
1
3
3
+
-
(
1
1
+
-
1
)
(
1.54 eV), which in turn again slows down the D- A f
1
+
-
1
3
D -A ) process and allows ISC (D- A f D- A) to compete
3
+
-
effectively. As a result, the long-lived (D -A ) state is
populated (Figure 5D).
Acknowledgment. The authors would like to thank H. J.
van Ramesdonk for his contribution to the transient absorption
experiments. The expert assistance of Ing. D. Bebelaar in
realization and operation of the SPC setup is gratefully
acknowledged. We thank A. van Laar for the synthesis of 3b
and 4.
(
(
(
(
(
(
(
60) Marcus, R. A. J. Chem. Phys. 1956, 24, 966.
61) Marcus, R. A. J. Chem. Phys. 1956, 24, 979.
62) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155.
63) Marcus, R. A. J. Chem. Phys. 1965, 43, 679.
64) Zhu, Y.; Schuster, G. B. J. Am. Chem. Soc. 1993, 115, 2190.
65) Hermant, R. M. Ph.D. Thesis, University of Amsterdam, 1990.
66) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1980, 86, 401.
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