Intramolecular energy transfer in a tetra-coumarin perylene system: Influence of solvent and bridging unit on electronic properties

Article abstract of DOI:10.1039/b711681k

The synthesis and characterisation of a novel coumarin donor-perylene bisimide acceptor light-harvesting system is reported, in which an energy-transfer efficiency of >99% is achieved. Comparison of the excited-state properties of the donor-acceptor syste

Full text of DOI:10.1039/b711681k

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Intramolecular energy transfer in a tetra-coumarin perylene system: influence
of solvent and bridging unit on electronic properties†
Johannes H. Hurenkamp,a Wesley R. Browne,a Ramu¯nas Augulis,b Audrius Pugzˇlys,‡b
a
Paul H. M. van Loosdrecht,b Jan H. van Esch*§ and Ben L. Feringa*a
Received 31st July 2007, Accepted 30th August 2007
First published as an Advance Article on the web 17th September 2007
DOI: 10.1039/b711681k
The synthesis and characterisation of a novel coumarin donor–perylene bisimide acceptor
light-harvesting system is reported, in which an energy-transfer efficiency of >99% is achieved.
Comparison of the excited-state properties of the donor–acceptor system with model compounds
revealed that although the photophysical properties of the perylene bisimide acceptor unit are affected
considerably by the nature of the substituent at the imide positions and the solvent employed,
through-bond interaction between the donor and acceptor units is negligible. Energy transfer in the
present system can be described as occurring via a through-space energy-transfer mechanism. Careful
consideration of the redox properties of the donor relative to the acceptor units allows for avoidance of
potentially deleterious excited-state electron-transfer processes.
bond. Furthermore, the porphyrin acceptor proved to be unstable
under the intense irradiation conditions required to saturate the
Introduction
Energy-transfer phenomena, such as those central to photosys-
system.
tems I and II,1 and, in particular, in dye-based photovoltaic
Here, we report the design and photophysical characterisation
systems,2 are of increasing importance in the drive to apply
of a coumarin donor–perylene bisimide acceptor system, which
molecular systems as components in photonic devices and the
enables efficient light harvesting (Fig. 1). The perylene bisimide
ever increasing pressure to develop CO2-neutral energy-generation
core is an excellent alternative for the porphyrin acceptor due to
technologies. Nature has served as a source of inspiration in
its high fluorescence quantum yield, redox properties, stability and
understanding the basic requirements for building efficient energy-
the ability of perylene bisimides to engage in both electron- and
transfer systems,1c,d in particular in mimicking aspects of the
complex architecture of the light-harvesting complexes PS I and PS
II. In developing synthetic systems in which to study energy trans-
fer but under low and high photon fluxes (so called multiphoton-
excitation conditions) it is essential that the components employed
are compatible energetically not only for efficient energy transfer
but also to avoid potentially deleterious photochemistry, e.g.
irreversible photoinduced electron transfer.3
Recently, we reported a tetra-coumarin–porphyrin-based den-
dritic system, which shows efficient intramolecular energy
transfer.4 However, the 7-methoxycoumarin-3-carboxylic acid
selected for the tetra-coumarin–porphyrin system showed a de-
creased quantum yield of fluorescence when coupled to an amine,
due to direct conjugation of the amide with the coumarin double
energy-transfer processes.5,6 These properties make them attractive
for application in photonic devices and as substitutes for inorganic
phosphorescent systems. Indeed, the similarity of both electronic
and redox properties of perylene bisimides with the paradigm
complex [Ru(bpy)3]2+ is remarkable.7 The planarity of perylene
bisimides enables the formation of H- and J-aggregates.8 Such ag-
gregation behaviour is advantageous for supramolecular systems,9
in achieving gelation,10 and in liquid-crystalline behaviour.11
However, in studying unimolecular processes, such as energy and
electron transfer in dendritic systems, aggregation is less desirable.
In recent years, substitution in the bay area of the perylene bisimide
has proven to be effective in inhibiting aggregation and as a
consequence increasing solubility dramatically. Substitution in the
bay area can be employed to connect the perylene bisimide unit
to donor units also,12 thereby combining increased solubility with
increased functionality. In the present contribution substitution
at the bay area is employed, as mentioned above, to facilitate
solubility. Whilst other substituents, specifically energy-donor
components, are introduced at the bisimide positions to provide
the desired functionality.
aStratingh Institute for Chemistry, Faculty of Mathematics and Natural
Sciences, University of Groningen, Nijenborgh 4, 9747AG, Groningen, The
Netherlands. E-mail: j.h.vanesch@tudelft.nl, b.l.feringa@rug.nl; Fax: +31
503 634 296; Tel: +31 503 634 235
bZernike Institute for Advanced Materials, Faculty of Mathematics and Nat-
ural Sciences, University of Groningen, Nijenborgh 4, 9747AG, Groningen,
The Netherlands
The perylene bisimide core has been employed successfully in
single- and multi-step donor–acceptor arrays, and has been shown
to be a good acceptor with high stability as demonstrated by the
groups of Fre´chet,13 Mu¨llen,14 De Schryver14 and Wu¨rthner.15
The choice of donor unit is critical for the study of energy trans-
fer in antenna systems, specifically Fo¨rster or resonance energy
transfer. Importantly, the redox properties of the donor and the
† Electronic supplementary information (ESI) available: Experimental
procedures, molecular orbital diagrams and 1H NMR spectra of 9, 12a–c.
See DOI: 10.1039/b711681k
‡ Present address: Photonics Institute, Vienna University of Technology,
Gusshausstrasse 27/387, 1040, Vienna, Austria
§ Present address: Delft ChemTech, Technical University Delft,
Julianalaan 136, 2628 BL, Delft, The Netherlands
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Fig. 1 Schematic representation of the convergent approach to the construction of the coumarin-perylene bisimide donor–acceptor system.
acceptor must not facilitate excited-state electron-transfer between
donor and acceptor.6c To match these requirements, a highly
fluorescent and stable, coumarin-based, donor was selected.1b
The 7-methoxycoumarin-3-acetic acid 1 employed in this study
was chosen to be compatible with the perylene bisimide core, in
terms of both electronic and redox properties. Furthermore, the
carboxylic acid functionality facilitates the use of amide chemistry.
A further important consideration lies in the properties of
the bridging unit between the donor and acceptor components
that enable reasonable control of the donor–acceptor separa-
tion and, to a lesser extent, orientation. Dendritic systems, i.e.
large regularly branched molecules,16 are of special interest as
candidates in energy-transfer systems, as demonstrated by the
groups of Balzani,17 Fre´chet13 and Wasielewski.5 Dendrimers
offer a possibility of arranging donor and acceptor units and of
controlling communication between these chromophores, and it
is this covalent-tethering approach, which is taken in the present
study. In the present study, which focuses on through-space energy
transfer, the bridging unit should not allow through-bond energy-
transfer processes to occur. Therefore, piperazine and amides
were used as spacer groups to disrupt through-bond electronic
communication between the donor and acceptor units.
amino)isophthalic acid,19 7,20b 8,20c and 920d,e were prepared accord-
ing to literature procedures. Details of the experimental procedure
for the synthesis of 10, 12a and 12b, and of the measurements
performed can be found in the ESI†.
3-Acetic acid-7-methoxycoumarin (1)
2-Hydroxy-4-methoxybenzaldehyde (25 g, 0.16 mol) and succinic
anhydride (50 g, 0.50 mol) were placed in a 250 ml three-necked
round-bottom flask fitted with a reflux condenser. The solid
mixture was heated on a metal plate to 90 ◦C and stirred for
30 min. The melt was then heated to 190 ◦C. Anhydrous succinic
acid, disodium salt (38 g, 0.23 mol) was added in small portions
over 4 h. The hot melt was poured into 10% HCl (aq) and left
overnight. The yellowish precipitate was filtered and washed with
water until neutral. The residue was dissolved in 5% NaHCO3
(aq) and filtered. The filtrate was added to cold 15% HCl (aq) and
left overnight. The precipitate was filtered and the residue washed
with H2O, dried, and recrystallised from H2O–ethanol yielding
brownish crystals (8.28 g, 35.8 mmol) in 21,8% yield. m.p. 177.5–
177.9 ◦C. 1H NMR (400 MHz, DMSO-d6) d = 12.47 (s, 1H), 7.90
(s, 1H), 7.60 (d, J = 8.6 Hz, 1H), 7.01 (d, J = 2.4 Hz, 1H), 6.95 (dd,
J = 8.6, 2.5 Hz, 1H), 3.85 (s, 3H), 3.45 (s, 2H). 13C NMR (50 MHz,
DMSO-d6) d = 171.41 (s), 161.89 (s), 160.86 (s), 154.58 (s), 141.74
(d), 128.95 (d), 119.23 (s), 112.41 (d), 112.38 (s), 100.42 (d), 55.81
(q), 35.60 (t). MS(EI) for C12H10O5 m/z 234 [M+], HRMS calcd
for C12H10O 234.053, found 234.053.
The polarity of the amide bonds provides an opportunity to
examine the effect of solvent polarity on the conformation of
the dendritic structure and also its effect on the energy-transfer
efficiency. In this paper we present the design and synthesis
of an efficient donor–acceptor system (Fig. 1), together with
a photophysical investigation of both the donor and acceptor
components and of the assembled donor–acceptor system.
7-Methoxy-3-(2-oxo-2-(N-Boc-piperazine)-1-yl-ethyl)coumarin
(1a)
N,Nꢀ-Carbonyldiimidazole (CDI) (1.4 g, 8.5 mmol) was added
to a suspension of 3-acetic acid-7-methoxycoumarin (2.0 g,
8.5 mmol) in dry CH2Cl2. The reaction mixture was stirred at room
temperature (RT) under N2 until CO2 evolution was complete,
and stirring was continued for another 30 min. N-Boc-piperazine
(1.57 g, 8.5 mmol) was added and the reaction mixture was stirred
Experimental
Uvasol-grade solvents (Merck) were employed for all spec-
troscopic measurements. All reagents employed in synthetic
procedures were of reagent grade or better, and used as re-
ceived unless stated otherwise. N-Boc-piperazine,18 5-(N-Boc-
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under N2 at RT overnight. The reaction mixture was extracted
twice with 1 M HCl (aq), twice with 5% NaHCO3 (aq), and
once with brine. The solution was dried over Na2SO4 and the
solvent evaporated. The crude mixture was purified using column
chromatography (2% MeOH in CH2Cl2, SiO2, Rf = 0.4) giving
a yellow solid (3.13 g, 7.76 mmol) in 91.8% yield. m.p. 164.0–
164.4 ◦C. 1H NMR (400 MHz, CDCl3) d = 7.65 (s, 1H), 7.32 (d,
J = 8.6 Hz, 1H), 6.80 (dd, J = 8.5, 2.4 Hz, 1H), 6.78 (d, J =
2.3 Hz, 1H), 3.83 (s, 3H), 3.58 (dd, J = 9.0, 5.0 Hz, 4H), 3.56 (s,
2H), 3.48-3.42 (m, 2H), 3.42-3.37 (m, 2H), 1.43 (s, 9H). 13C NMR
(100.6 MHz, CDCl3) d = 168.4 (s), 162.4 (s), 161.9 (s), 155.1 (s),
154.5 (s), 141.8 (d), 128.5 (d), 119.3 (s), 112.6 (s), 112.9 (d), 100.5
(d), 80.2 (s), 55.7 (q), 45.96 (t), 41.8 (t), 34.0 (t), 28.3 (q). MS(EI) for
C21H26N2O6 m/z 402 [M+], HRMS calcd for C21H26N2O6 402.179,
found 402.180.
filtered off and the residue washed with 1 M HCl (aq) and water.
The residue was taken up into MeOH–CH2Cl2 1 : 1, dried over
Na2SO4, and the solvent evaporated. The crude compound was
purified by column chromatography (4% MeOH in CH2Cl2, SiO2)
1
yielding the purple product 12c (40 mg, 0.016 mmol, 16%). H
NMR (400 MHz, CDCl3) d = 8.26 (s, 4H), 7.65 (s, 4H), 7.63 (t,
J = 1.5 Hz, 2H), 7.43 (d, J = 1.3 Hz, 4H), 7.35 (d, J = 8.6 Hz,
4H), 7.23 (d, J = 8.9 Hz, 8H), 6.85–6.77 (m, 16H), 3.86 (s, 12H),
3.83-3.38 (m, 40H), 1.26 (s, 36H). 13C NMR (100.6 MHz, CDCl3)
d = 169.5 (s), 169.4 (s), 164.2 (s), 163.5 (s), 163.0 (s), 157.2 (s),
156.2 (s), 153.7 (s), 148.6 (s), 143.1 (d), 137.5 (s), 136.2 (s), 134.2
(s), 130.7 (d), 129.6 (d), 127.8 (d), 123.2 (s), 122.1 (s), 121.4 (d),
120.8 (s), 120.4 (s), 120.3 (d), 113.9 (d), 113.8 (d), 101.6 (d), 56.8
(q), 48.7 (d), 47.1 (d), 43.3 (d), 35.4 (d), 35.2 (d), 32.5 (q), 30.7 (s).
MALDI-TOF MS (Mw = 2447.91) m/z = 2470.46 [M + Na+].
(5-tert-Butoxycarbonylamino)isophthalic acid
bis(2,5-dioxopyrrolidin-1-yl) ester (13)
General deprotection method for BOC-protected amines
The Boc-protected amine was stirred in a mixture of 1 : 1 CH2Cl2–
CF3COOH for 4 h. An equal amount of water was added and the
mixture was neutralised by addition of solid NaHCO3, after which
the aqueous layer was separated and the organic layer was washed
with saturated NaHCO3 solution. Subsequently the organic layer
was dried over Na2SO4 and the solvent evaporated. The resulting
product was used in subsequent steps without further purification.
A
suspension of 5-(N-Boc-amino)isophthalic acid (1.0 g,
3.5 mmol) and N-hydroxysuccinimide (886 mg, 7.7 mmol) in dry
◦
THF under a dinitrogen atmosphere was stirred at 0 C. N,Nꢀ-
Dicyclohexylcarbodiimide (1.6 g, 7.7 mmol) was added, and the
solution was stirred overnight at RT. The suspension was filtered
and the filtrate collected. The solvent was evaporated and the
resulting solid was recrystallised from 2-propanol yielding a white
1
powder (853 mg, 1.77 mmol, 51%). m.p. 205.1–205.6. H NMR
(3,5-Bis{4-[2-(7-methoxy-2-oxo-2H-chromen-3-
yl)acetyl]piperazine-1-carbonyl}phenyl)carbamic acid tert-butyl
ester (4)
(400 MHz, CDCl3) d = 8.50 (s, 1H), 8.44 (s, 1H), 6.99 (s, 2H), 2.90
(s, 8H), 1.52 (s, 9H). 13C NMR (50.3 MHz, CDCl3) d = 169.1 (s),
160.7 (s), 152.3 (s), 140.4 (s), 127.1 (s), 126.4 (d), 125.4 (d), 82.2
(s), 28.4 (q), 25.9 (t). MS(EI) for C23H33N3O4 m/z 475.1 [M+].
BOC-protected 7-methoxy-3-(2-oxo-2-(N-Boc-piperazine)-1-yl-
ethyl)coumarin (1a) was deprotected using the general method
described above, and (2.0 g, 6.6 mmol) of the deprotected com-
pound was suspended in THF with 5-(N-Boc-amino)isophthalic
acid (0.66 g, 2.4 mmol). 4-(4,6-Dimethoxy-1,3,5-triazin-2-yI)-4-
methylmorpholinium chloride (DMTMM) (2.0 g, 7.2 mmol) was
added and the suspension was stirred overnight. The solvent was
evaporated and the crude mixture was purified using column
chromatography (4% MeOH in CH2Cl2, SiO2, Rf = 0.4), yielding
a light yellow solid (0.87 g, 1.02 mmol, 43%) m.p. 190.5–191.2 ◦C.
1H NMR (400 MHz, CDCl3) d = 7.66 (s, 2H), 7.53 (s, 2H), 7.34
(d, J = 8.6 Hz, 2H), 7.08 (s, 2H), 6.97 (s, 1H), 6.82 (dd, J = 8.6,
2.4 Hz, 2H), 6.79 (s, 2H), 3.84 (s, 6H), 3.81–3.36 (m, 20H), 1.49
(s, 9H). 13C NMR (50 MHz, CDCl3) d = 169.2 (s), 168.5 (s), 162.4
(s), 161.9 (s), 155.1 (s), 152.5 (s), 142.0 (d), 139.6 (s), 136.2 (s),
128.5 (d), 119.6 (d), 119.2 (s), 118.3 (d), 112.8 (s), 112.7 (d), 100.4
(d), 80.9 (s), 55.7 (q), 47.5 (t), 46.0 (t), 41.9 (t), 34.1 (t), 28.2 (q).
MALDI-TOF MS (Mw = 849.32) m/z = 872.51 [M + Na+].
[3,5-Bis(piperidine-1-carbonyl)phenyl]carbamic acid tert-butyl
ester (14)
A suspension of 13 (2.0 g, 4.2 mmol), piperidine (1.1 g, 12,6 mmol)
and triethylamine (1.3 g, 21.6 mmol) in CH2Cl2 (100 ml) was
stirred overnight under a dinitrogen atmosphere. The suspension
was subsequently washed with a 1 M HCl (aq) (twice), a saturated
aqueous NaHCO3 solution (twice), water and finally with brine.
The organic layer was dried with Na2SO4, and the solvent
evaporated. The solid residue was dissolved in MeOH, after which
the product was precipitated by dropwise addition to 1 M HCl
(aq). The white precipitate is filtered, washed with water and dried
in a vacuum oven at 40 ◦C, yielding a white powder (1.18 g, mmol,
57%). 1H NMR (400 MHz, CDCl3) d = 9.66 (s, 1H), 7.51 (s, 2H),
6.87 (s, 1H), 3.56 (s, 4H), 3.25 (s, 4H), 1.48 (s, 9H), 1.38–1.65 (m,
12H). 13C NMR (50.3 MHz, CDCl3) d = 169.4 (s), 152.8 (s), 139.7
(s), 137.5 (s), 119.0 (d), 118.0 (d), 80.9 (s), 49.0 (t), 43.4 (t), 28.5 (q),
26.7 (t), 25.8 (t), 24.7 (t). MS(EI) for C23H33N3O4 m/z 415 [M+],
HRMS calcd for C12H10O 415.248, found 415.247.
N,Nꢀ-(3,5-Bis{4-[2-(7-methoxy-2-oxo-2H-chromen-3-
yl)acetyl]piperazine-1-carbonyl}phenyl)-1,6,7,12-tetrakis[4ꢀ-tert-
butylphenoxy]-3,4:9,10-perylenetetracarboxylic diimide (12c)
Results and discussion
In a 100 ml round-bottom flask fitted with a reflux condenser 10
(100 mg, 0.10 mmol), 5 (225 mg, 0.30 mmol) (obtained after depro-
tection of 4 via the standard procedure) and dimethylacetamide
(20 ml) were heated and stirred at 120 ◦C under dinitrogen. After
5 days the reaction was judged complete (by TLC, 10% MeOH
in CH2Cl2, SiO2). The reaction mixture was poured into 50 ml
1 M HCl (aq) and left to stand overnight. The precipitate was
The construction of the donor–acceptor systems employs se-
quential amide-coupling steps in a convergent manner. The
coumarin donor units were connected to branching unit 3
via two consecutive amide-coupling reactions (Fig. 2). First,
the acetic acid-functionalised coumarin 1 was connected to a
mono-Boc piperazine, after which the protecting group was
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removed to give the amine-functionalised coumarin 2. The
functionalised coumarin 2 was then coupled to 5-(N-Boc-
amino)isophthalic acid using 4-(4,6-dimethoxy-1,3,5-triazin-2-
yI)-4-methylmorpholinium chloride (DMTMM) with an overall
yield of 20%, after which the protecting group was removed
yielding 5 (Fig. 2), which was used without further purification.
butylphenoxy)-substituted bisanhydride 10 after acidic work-up
in 46% yield over 4 steps (Fig. 3).
Condensation of 10 with an excess of 5 provided, after purifica-
tion, the tetra coumarin perylene bisimide 12c in 16% yield. The
model compounds 12a and 12b were synthesised following similar
procedures (Fig. 4).
The compounds were purified by column chromatography and
1
characterised by H and 13C NMR spectroscopy and MALDI-
TOF mass spectroscopy (see experimental section for details).
Electronic properties
The absorption spectra of the perylene bisimide model compounds
and 12c in CH2Cl2 are shown in Fig. 5 and the data are summarised
in Table 1. Examination of the spectra of the substituted perylene
bisimides (Fig. 5) reveals a bathochromic shift with decreasing
electron donating strength of the substituent. The visible absorp-
tion spectrum (k > 400 nm) of 12b is almost identical to that of
12c. This indicates that the electronic influence of the substituent
is limited to the bridging unit itself and any influence of the
coumarin component observed is unlikely to be through bond in
character.
Emission spectra (kex = 450 nm) of the model compounds
(Fig. 6) and 12c show a trend analogous to that observed in the
absorption spectra; a slight bathochromic shift is observed with
decreasing electron-donating strength of the substituent (Table 1).
As a consequence of the similarity of the redox (vide infra),
absorption and emission properties of 12b and 12c, the former
Fig. 2 Synthesis of the dicoumarin branch 5: (a) CDI, CH2Cl2,
N-Boc-piperazine; (b) CH2Cl2, CF3COOH, 4 h; (c) DMTMM, THF, 18 h;
(d) CH2Cl2, CF3COOH, 4 h.
The core unit was prepared using literature procedures.20 The
perylene bisanhydride 6 was transformed to the n-butyl bisimide
7 by condensation with n-butylamine (Fig. 3). The bisimide was
tetra-chlorinated using sulfuryl chloride in nitrobenzene to yield
8. Aromatic substitution using 4-tert-butylphenol provided the
tetraphenol-substituted perylene bisimide 9. The bisimide was
saponified with potassium hydroxide to give the tetra(4-tert-
Fig. 3 Synthesis of the core acceptor unit (a) n-butylamine, quinoline, 220 ◦C, 6 h; (b) SO2Cl2, I2, PhI, PnNO2, reflux, 80 ◦C, 20 h; (c) 4-tert-butylphenol,
K2CO3, NMP, 130 ◦C, 3 days; (d) KOH, H2O, t-BuOH, reflux, 24 h; (e) HCl (aq).
Fig. 4 Synthesis of 12a–c: (a) toluene, 120 ◦C, 5 d; (b) DMA, 120 ◦C, 5 d.
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Table 1 Absorption and emission spectra of 4, 9 and 12a–ca
Absorption
Emission
Lifetime
kmax/nm (103 e/cm−1 M−1
)
kmax/nm (Ufl)
s/nsb
1a
322(18.3)
324(31.3)
393d (0.46d)
1.38d
1.51d
6.66f
6.46f
6.30f
5.94f
n.d.
4
394d (0.50d)
9
266(40.6), 286(49.6), 451(16.7), 539(26.7), 577(43.1)
265(43.0), 290(43.6), 452(16.9), 540(28.1), 580(45.7)
289(41.9), 455(15.7), 544(28.1), 584(44.6)
295(63.6), 322(61.5), 458(14.5), 546(25.7), 587(41.9)
294, 321, 455, 544, 584
608e (0.66i)
12a
12b
12c
4 + 12cc
612e (0.59 i)
616e (0.57 i)
394d, 618d (<0.03,d,g 0.47 h,i
394d, 619d (n.d.)
)
a Recorded in CH2Cl2 at RT. b Emission lifetime, experimental uncertainty ∼2.5%. c 2 : 1 mixture. d kex = 322 nm. e kex = 450 nm. f kex = 420 nm. g Residual
coumarin emission. h Direct excitation of the perylene bisimide unit. i kex = 539 nm.
absorption spectrum of 12c (Fig. 7). The absorption spectrum
of the model mixture of 12b and 4 is almost identical to that
of 12c, with no significant shifts in either the red or the blue
region, indicating little or no communication between coumarin
and perylene bisimide is present. The location of the maximum
at k = 324 nm and the intensity of the coumarin 4 absorption
coincide with the absorption of the coumarin component of 12c.
Similarly the emission kmax of the model coumarin and the residual
emission in 12c compare well (vide infra).
Fig. 5 Absorption spectra of 9 (—), 12a (---), 12b ( · · · ) and 12c (-·-) in
CH2Cl2 at RT. Inset = expansion of the 560 to 600 nm region.
Fig. 7 Absorption spectra of 12c (—), the 1 : 2 mixture of 12b and 4 (---),
and the spectrum of 4 ( · · · ), spectra were recorded in CH2Cl2 at RT, the
spectra of 12c and the mixture of 4 and 12b are normalised to the perylene
bisimide absorption maximum ∼590 nm.
The fluorescence lifetimes of the model perylene bisimide
compounds 9, 12a and 12b (kex = 420 nm), substituted with
butyl, phenyl and bridge model 11, respectively (Fig. 4), show
a modest but significant decrease in excited-state lifetimes and
are accompanied by a concomitant decrease in the fluorescence
quantum yield (Table 1). The trend observed is comparable to
the bathochromic shift observed in both absorption and emission
spectra. The decrease in emission lifetime is not unexpected and
can be rationalised on the basis of the energy-gap law,21 which
predicts a decrease in the nonradiative emission lifetime with a
decrease in ground–excited-state energy gap, however it should be
noted that the overall difference in the ground and emissive-excited
states in this series is 250–300 cm−1 in total.22
Fig. 6 Emission spectra of 9 (—), 12a (---) and 12b ( · · · ) in CH2Cl2 at
RT (kex = 450 nm), showing the decrease in the relative quantum yield of
emission.
compound was chosen as a model for the core acceptor unit in
photophysical studies.
As for the acceptor unit, a suitable model for the donor part must
be identified in order to examine the energy-transfer processes
within 12c. Due to overlap of absorption in 12c of the coumarin
and perylene bisimide components confirmation of the suitability
of 4 as a model compound was obtained from comparison of
the spectrum of a 2 : 1 molar mixture of 4 and 12b with the
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Table 2 Redox potentials of 9 and 12a–c
to a slightly greater extent than the HOMO, and thereby show a
decrease of the HOMO–LUMO gap overall.
Compound
Oxidation/Va
Reduction/Va
DV/Vb
Solvent-dependence of spectroscopic properties
9
1.25
1.28
1.31
1.33
−0.77
−0.72
−0.66
−0.64
−0.92
2.02
2.00
1.97
1.97
12a
12b
12c
−0.88
−0.83
−0.81
Absorption and emission spectra of all compounds were obtained
in acetone, dichloromethane and chloroform. For the coumarin
model 4 both absorption and emission spectra were identical in
dichloromethane and chloroform (aggregation was observed in
acetone).
a Differential pulse voltammetry. b DV is the separation between E1/2 of
the first oxidation and the first reduction (V vs. SCE, in CH2Cl2–0.1 M
TBAPF6).
For the perylene bisimide-containing compounds, however, con-
siderable solvent-dependence in both the emission and absorption
spectra was observed (Fig. 9 and Table 4). The spectra show
a red-shift in both absorption and emission between acetone,
dichloromethane and chloroform. The origin of this solvent-
dependence can be assigned to the interaction of the imide
carbonyls of the perylene bisimide unit with the solvent, and
is similar to the effect observed upon substitution at the imide
nitrogen (vide supra). This solvent effect highlights the influence of
the imide carbonyls on the HOMO–LUMO levels of the perylene
bisimide core. The nature of the interaction, vis a` vis HOMO vs.
LUMO stabilisation, can be estimated from the redox potentials
of the compounds in these solvents.
Redox properties
The redox potentials of perylene bisimides (9, 12a–c, Table 2) were
measured by differential pulse and cyclic voltammetry in CH2Cl2–
0.1 M TBAPF6 between +1.6 V and −1.2 V vs. SCE. At anodic
potentials a reversible one-electron redox peak (E1/2 +1.25 V to
+1.33 V vs. SCE) is observed for all perylene bisimide-based
compounds (Fig. 8). Similarly at cathodic potentials two reversible
one-electron redox waves are observed, with the separation
between the first process (E1/2 −0.64 V to −0.77 V vs. SCE) and
the second process (E1/2 −0.81 V to −0.92 V vs. SCE) showing
only minimal dependence (∼5 mV) on the substituent employed
at the imide position. Comparison of 12c with the model perylene
bisimide systems confirms that both the first oxidation and the first
and second reduction processes are based on the perylene bisimide
core.6b,20b Within the potential window examined (+1.6 to −1.6 V
vs SCE) no redox activity was observed for the coumarin or amide
components.
Fig. 9 Absorption and emission spectra of 12b in acetone (–), CH2Cl2
(---) and chloroform ( · · · ) at RT (kex = 450 nm, spectra normalised for
clarity).
The redox properties of the perylene bisimide compounds in
dichloromethane and chloroform are shown in Table 3. It is
Fig. 8 Cyclic voltammetry of 9 and 12a–c in CH2Cl2/0.1 M TBAPF6 vs
SCE. The current is offset for clarity.* indicates the open circuit potential.
Table 3 Redox potentialsaof 9 and 12a–b in dichloromethane and
chloroform
The correlation between the HOMO–LUMO energy gap
determined electrochemically and spectroscopically is well
established,23 providing both the oxidation and reduction involve
the same chromophoric unit. The separation (DV) between first
oxidation and first reduction decreases with decreasing electron-
donating ability of the substituent. This indicates a decrease in the
HOMO–LUMO energy gap. For 12b and 12c, the similarity in the
separation (DV) indicates a comparable influence on the perylene
bisimide core by both imide substituents. Comparison of 9 and 12b
indicates that electron-withdrawing groups destabilise the LUMO
Dichloromethane
Chloroform
E1/2
DV/V
E1/2
DV/V
9
1.25, −0.77, −0.92 2.02
1.28, −0.87, −1.03 2.15
1.32, −0.80, −0.99 2.12
1.32, −0.72, −0.91 2.04
12a 1.28, −0.72, −0.88 2.00
12b 1.31, −0.66, −0.83 1.97
12c
1.33, −0.64, −0.81 1.97
a Determined by differential pulse voltammetry (V vs. SCE, in respective
solvent–0.1 M TBAPF6)
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Table 4 Solvent-dependence of the absorption and emission maxima of
the perylene bisimide emission (k ∼ 580 nm) of 9 and 12a–c
Absorptiona (kmax)/nm
Emissiona,b (kmax)/nm
Acetone
CH2Cl2 CHCl3
Acetone
CH2Cl2
CHCl3
9
568
568
571
573
577
580
584
587
585
588
590
593
603
603
603
606
608
614
616
618
620
622
623
626
12a
12b
12c
a Measurements taken at RT b kex = 450 nm.
apparent from the shift in first-reduction potential (vs. SCE) going
from dichloromethane to chloroform for each separate compound
(9, 12a–c) that the LUMO level is affected by the solvent to only a
slightly larger extent than the HOMO level. A key aspect of energy
transfer is the relative importance of through-bond and through-
space contributions to the overall interaction between the donor
and acceptor units. Typically the through-bond contribution is
estimated by comparison of the spectroscopic properties of the
separate donor and acceptor component with those of 12c. It
is clear that although minor differences in the absorption and
emission spectra between the 2 : 1 mixture (of 4 and 12b) and 12c
are observed, these differences are comparable to the effect of local
solvent environment.
The molecular geometry and the orbital-energy diagrams of 9,
12a and 12b were calculated using the hybrid Hartree–Fock density
functional method (B3LYP/6-31G(d)).24 The tert-butyl groups on
the phenol bay substituent were replaced by methyl groups, for
9 instead of a butyl group a methyl was used and for 12b the
piperidine groups were replaced by dimethylamine substituents,
this was done to increase symmetry and to reduce calculation cost,
all replacement substituents were selected to have minimal impact
on the electronic structure. Molecular orbital diagrams of 9, 12a
and 12b show a considerable contribution of the imide carbonyls
to both the HOMO and LUMO of the model compounds (ESI†).
It is clear that the imide substituents are not involved in the
frontier orbitals of the perylene compounds examined despite
holding a strong influence over the electronic character of the
carbonyl bonds. This suggests that the effect of both the solvent
and the imide substituents on the electronic properties is due to
the perturbation of electron density on the carbonyl groups.
Fig. 10 Absorption and fluorescence spectra of 12c (abs —) and 4
(abs ---, fl · · · , kex = 322 nm) normalised to 12c absorption maximum
at k = 587 nm in CH2Cl2 at RT.
that the spectroscopic properties of the perylene bisimide core are
unaffected by the covalent attachment of the coumarin. Similarly,
upon excitation at kex = 322 nm, corresponding to the kmax of the
coumarin absorption, both the mixture and 12c show emission at
kmax = 394 nm (coumarin emission) and 616 nm (perylene bisimide
emission). The intensity of the coumarin emission in both systems
is very different, however. The reduced intensity of the coumarin
emission, observed for 12c, indicates quenching of the coumarin
emission is taking place (the coumarin Ufl decreases from 0.5 to
<0.03, Table 1). By contrast, the intensity of the perylene bisimide
emission of 12c is increased compared to that in the model system.
Hence intramolecular energy transfer from the coumarin to the
perylene bisimide core is taking place in 12c.
Energy transfer
The absorption spectrum of the coumarin substituted branch
4 in CH2Cl2 at RT shows an absorption maximum at k =
324 nm (Fig. 7). At this wavelength the absorption of the perylene
bisimide model is low (Fig. 5), allowing for direct excitation of the
donor unit with minimal direct excitation of the perylene bisimide.
In order for energy transfer to occur via a Fo¨rster energy-transfer
mechanism, overlap of the donor-emission spectrum and acceptor-
absorption spectrum is required. The emission of 4 in CH2Cl2 at
RT shows a maximum at 394 nm and overlaps with the absorption
of the perylene bisimide acceptor unit (Fig. 10).
Fig. 11 Emission spectra corrected for the absorption at kex = 322 nm of
the model mixture (2 : 1 mixture of 4 (3 × 10−5 M) and 12b (1.5 × 10−5 M),
kex = 322 nm, —), and 12c (0.5 × 10−5 M, kex = 322 nm, ---) in CH2Cl2 at
RT.
The emission spectra of a 2 : 1 mixture of 4 and 12b, and of
12c were measured in CH2Cl2 at RT (Fig. 11). Excitation at kex
450 nm results in perylene bisimide emission of similar intensity
at kem = 616 nm for both systems (not shown). This confirms
=
The absence of intermolecular energy transfer was confirmed
through excitation spectroscopy (Fig. 12). The excitation spectrum
of 12c matches the corresponding absorption spectrum closely
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coumarin donor units or the perylene acceptor core itself). Hence
the rapid decay of the coumarin emission and the concomitant
rise time of the perylene bisimide component observed by time-
resolved spectroscopy provides a strong indication that energy
transfer from the coumarin to the perylene bisimide is efficient.
Conclusions
In understanding and characterising energy-transfer processes it
is essential that the models used for comparison with the dendritic
system are suitable. The imide substituent is expected to have a
negligible effect on the properties of the perylene bisimide core;
however, it is clear from comparison of 9 with 12b that a significant
shift in the absorption spectrum results from a change in the
imide substituents. Differences between the absorption spectra
of 12c and model compounds 9, 12a and 12b become less for the
imide-substituted models, which are structurally most similar to
branch unit 4 used in 12c. This is quite evident when considering
the trends observed in the absorption spectra, but can also be
seen when comparing the redox properties. Hence 12b is the most
suitable model compound for the acceptor unit of the donor–
acceptor system 12c. The remaining differences (∼2 nm shift)
can be rationalised by considering the small differences in local
environment, most likely caused by the branches, and are much
smaller than differences observed with different solvents (i.e. upon
changing from dichloromethane to chloroform, a ∼10 nm shift is
observed).
These small differences suggest that through-bond orbital
interaction is minimal if indeed it is present and hence through-
bond energy transfer is unlikely. The model mixture of 4 and
12b also shows that the energy transfer is not due to trivial
(intermolecular) energy transfer, since irradiation at the kmax of
4 shows no increase of the perylene bisimide emission compared
to a solution of 12b irradiated at the same wavelength. Irradiation
of 12c at the kmax of 4 shows a clear increase in fluorescence
of the perylene bisimide, indicating transfer of energy from
the coumarin donor to the perylene bisimide acceptor. This is
confirmed by comparison of the excitation spectra of 12c and
model mixture of 4 and 12b, which show that the energy originates
from the coumarin donor and not only from the residual perylene
bisimide absorption at that excitation wavelength (Fig. 12, the
coumarin kmax).
Fig. 12 Excitation spectra of 12b (—), 12c (---), and a 2 : 1 mixture of,
respectively, 4 and 12b ( · · · ) in CH2Cl2 at RT, normalised at the perylene
bisimide absorption maximum at 586 nm.
and, importantly, shows the contribution of the coumarin ab-
sorption to the perylene bisimide emission in 12c. Comparison of
the excitation spectra of 12b and the 2 : 1 mixture of 4 and 12b
shows that the free coumarin does not contribute to emission of
the perylene bisimide model compound. Hence, it is unlikely that
intermolecular energy transfer occurs in solution for 12c.
Preliminary time-resolved emission spectroscopy confirms that
energy transfer from the coumarin donor units to the perylene
bisimide acceptor core is fast (<15 ps, Fig. 13). The presence
of four donor coumarin units and the non-zero absorption of
the perylene bisimide unit itself in the near UV region opens the
possibility that several components of the system can be pumped
optically to an electronically excited state within the lifetime of the
perylene bisimide acceptor excited state, resulting in potentially
complex excited-state behaviour. Under the low excitation inten-
sity conditions employed here, however, statistically only one unit
in the array is excited at any one time (i.e., either one of the four
In the present report we have described the synthesis and
characterisation of a tetra coumarin donor–perylene bisimide
acceptor array. The energy transfer from the donor units to the
acceptor was found to be very efficient (>99%) and to take place
via a through-space mechanism. In addition to the high efficiency
of energy transfer, which is comparable to several related systems
reported previously,13,14,15 the donor4–acceptor array shows high
stability, and its redox properties indicate that no undesired,
and possibly deleterious, photoinduced electron transfer between
the donor and acceptor takes place. Importantly, recognition of
the sensitivity of the acceptor to both solvent properties and
the nature of the imide substituents is critical for excluding a
through-bond contribution to the energy-transfer mechanism. An
investigation25 of the picosecond energy-transfer dynamics and
the behaviour of the donor–acceptor system under multiphoton
excitation conditions is currently in progress.
Fig. 13 Time-resolved emission spectroscopy of 12c in CHCl3. irf-instru-
ment response function, kem 385 nm (coumarin unit), kem 620 nm (perylene
bisimide unit). Excitation wavelength kex = 325 nm.
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708; (b) C. Flors, I. Oesterling, T. Schnitzler, E. Fron, G. Schweitzer,
M. Sliwa, A. Herrmann, M. von der Auweraer, F. C. De Schryver,
K. Mu¨llen and J. Hofkens, J. Phys. Chem. C, 2007, 111, 4861; (c) M.
Cotlet, T. Vosch, S. Habuchi, T. Weil, K. Mu¨llen, J. Hofkens and F. De
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Acknowledgements
The authors thank Prof. J. G. Vos and Dr W. H. Henry for
assistance with TCSPC measurements. The Zernike Institute
for Advanced Materials (JHH, RA) and Nanoned (WRB) are
acknowledged for funding. This work is part of the research
program of the Stichting voor Fundamenteel Onderzoek der
Materie (FOM), which is financially supported by the Nederlandse
Organisatie voor Wetenschappelijk Onderzoek (NWO) (AP).
15 (a) L. Flamigni, B. Ventura, C. C. You, C. Hippius and F. Wu¨rthner,
J. Phys. Chem. C, 2007, 111, 622; (b) C. C. You, C. Hippius, M. Gru¨ne
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