FRET in orthogonally arranged chromophores

Article abstract of DOI:10.1002/ejoc.200800451

Perylene and benzoperylene carboxylic imides were arranged to form a bichromophoric dye with orthogonal electronic transition moments. Thus, exciton interactions between the two chromophores could be excluded, in spite of their proximity. However, quantitative Foerster-type energy transfer proceeds between the chromophores indicated by a fluorescence quantum yield close to unity even with hypsochromic excitation. Applications are discussed. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800451

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200800451
FRET in Orthogonally Arranged Chromophores
Heinz Langhals,*[a] Simon Poxleitner,[a] Oswald Krotz,[a] Tim Pust,[a] and Andreas Walter[a]
Dedicated to Prof. H. Nöth on the occasion of his 80th birthday
Keywords: FRET (Fluorescence Resonance Energy Transfer) / Fluorescence spectroscopy / Nitrogen heterocycles / Arenes /
Photophysics
Perylene and benzoperylene carboxylic imides were ar-
ranged to form a bichromophoric dye with orthogonal elec-
tronic transition moments. Thus, exciton interactions be-
tween the two chromophores could be excluded, in spite of
their proximity. However, quantitative Förster-type energy
transfer proceeds between the chromophores indicated by a
fluorescence quantum yield close to unity even with hyp-
sochromic excitation. Applications are discussed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
Förster-type fluorescence energy transfer (FRET)[1] is
gaining increasing interest in chemistry. FRET proved to be
a powerful tool for the indication of the proximity of chemi-
cal structures. Such information is required for the investi-
gation of molecular recognition. Thus, FRET systems are
becoming of increasing importance in biochemistry and ge-
neral analytics.[2] One may ask if there are limitations for
the FRET process by the orientation of the components,
because this would require special care for designing such
systems.
Perylene anhydride carboxylic imide 3[8] with the solubi-
lizing long-chain sec-alkyl group[9] at the nitrogen atom was
condensed with an excess amount of tetramethyl p-phenyl-
ene diamine to obtain amino dye 4. The methyl groups of
the phenyl spacer render the π systems of the chromophore
and the spacer orthogonal and, thus, electronically decou-
pled; further decoupling is brought about by the fact that
there are orbital nodes[10] in the HOMO and LUMO at
the linking nitrogen atom of 2 and 4, respectively. Amino
derivative 4 was condensed with anhydride 5[3] to obtain
bichromophoric dye 6 (Scheme 1). The three solubilizing
alkyl groups in 6 render the material soluble in spite of the
accumulation of aromatics. The electronic transition mo-
ments both in 1 and 2 are parallel to the six-membered ring
N–N-connection lines and, thus, are orthogonal in 6; this
was further verified by quantum chemical calculations[11] of
a simplified derivative and is shown in Figure 1. The or-
thogonality remains independent from a rotation around
the linking single bonds of the central p-phenylene unit in
6.
Results and Discussion
Benzoperylene hexacarboxylic trisimides 1[3] and per-
ylene tetracarboxylic bisimides 2[4] are ideal chromophores
for the investigation of the influence of orientation on
FRET, because there is only one electronic transition in the
visible region,[5] both in absorption and fluorescence spec-
troscopy, and this is polarized along the N–N axis of 1 and
2, respectively; dyads with 2 were developed[6] and further
progressed. As a novel concept, we arranged both the tran-
sition moments of 1 and 2, respectively, orthogonally in or-
der to clarify if FRET can be switched off; compare.[7] To
this end, an arrangement of the two chromophores with a
stiff spacer has to be synthesized.
[a] Department of Chemistry
LMU University of Munich
Butenandtstr. 13, 81377 Munich Germany
Fax: +49-89-2180-77640
E-mail: Langhals@lrz.uni-muenchen.de
Homepage: http://www.cup.uni-muenchen.de/oc/langhals/index.
html
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H. Langhals, S. Poxleitner, O. Krotz, T. Pust, A. Walter
SHORT COMMUNICATION
Figure 2. Absorption (thick line left hand) and fluorescence spec-
trum (thick line right hand) of 6 in chloroform. Thin lines from
left to right: absorption spectra of 1a and 2a and fluorescence spec-
trum of 2a in chloroform.
Figure 3. Gaussian analysis of the absorption (thick line left hand)
and the fluorescence spectrum (thick line right hand) of 6 in chloro-
form (400–750 nm). Bars: positions and intensities of the Gaussian
bands; thin lines: simulated spectra on the basis of the Gaussian
analysis; see Table 1.
Scheme 1. Synthesis of bichromophoric dye 6.
The line positions and intensities of the bathochromic
absorption of 6 and the fluorescence are very similar to the
corresponding bands of 2a indicating negligible interaction
of the two chromophores in 6 (see Table 1). This demon-
strates the exclusion of exciton interactions by orthogonal-
ity; compare ref.[7] The fluorescence quantum yield of 6 is
close to 100% if bathochromic absorbing chromophore 2 is
irradiated at 490 nm, as one may expect. Surprisingly, the
fluorescence quantum yield is also close to 100% if hy-
psochromically absorbing chromophore 1 is irradiated at
435 nm. The fluorescence of chromophore 1 is completely
Figure 1. Arrangement of the chromophores in 6 verified by quan-
tum chemical calculations of a simplified structure (see text). The
electronic transition moments are parallel to the N–N-connection quenched and only a bathochromic emission of chromo-
lines for each of the two chromophores and remain orthogonal
even after torsion of the chromophores around the single bonds of
phore 2 must proceed with a quantum yield close to 100%.
the central unit.
phore 2 is observed. Thus, the energy transfer to chromo-
This efficient energy transfer is surprising because of the
rather stiff orthogonal arrangement of the two chromo-
The UV/Vis absorption spectra of two chromophores 1 phores. Obviously, the orthogonal geometry of the two
and 2 in 6 are additive (see Figure 2). Exciton interactions chromophores cannot prevent efficient energy transfer. Vib-
are negligible for absorption, because the well-established ronic effects may be one reason, because the fluorescence
vibrational pattern of the chromophore of 1 and 2, respec- lifetime of 1 and 2 is about 5 ns[4] and, therefore, much
tively, remains unaltered (see Figure 2). Alterations in the longer than the time scale of vibration. The bending of the
spectra of 6 relative to those in the spectra of monochromo- C–N single bonds in 6 would break the orthogonal arrange-
phore 2 are even more precisely studied by means of ment of the chromophores and could open a pathway for
Gaussian analysis[12] (see Figure 3 and Table 1).
energy transfer.
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FRET in Orthogonally Arranged Chromophores
air (argon atmosphere) was heated at 105 °C for 4 h, cooled,
quenched while still warm with an acetic acid/2- HCl (1:1,
200 mL), collected by vacuum filtration (D4 glass filter), washed
with distilled water, dried in air at 110 °C for 16 h, and purified by
MPLC chromatography [silica gel, chloroform/ethanol (10:1), flux
30 mLmin–1]. Yield: 146 mg (39%) dark-red solid. M.p. Ͼ300 °C.
Table 1. Gaussian analysis of UV/Vis spectra in chloroform (400–
750 nm).
Dye
6 abs.
1a abs.
6 flu.
1a flu.
λmax(1)[a]
2 σ2(1)[b]
Emax(1)[c]
λmax(2)[a]
2 σ2(2)[b]
Emax(2)[c]
λmax(3)[a]
2 σ2(3)[b]
Emax(3)[c]
λmax(4)[a]
2 σ2(4)[b]
Emax(4)[c]
λmax(5)[a]
2 σ2(5)[b]
Emax(5)[c]
λmax(6)[a]
2 σ2(6)[b]
εmax(6)[c]
λmax(7)[a]
2 σ2(7)[b]
Emax(7)[c]
λmax(8)[a]
2 σ2(8)[b]
Emax(8)[c]
R[d]
527.6
0.125
0.984
513.7
0.082
0.239
490.4
0.280
0.607
466.0
0.185
0.779
453.8
0.133
0.221
436.7
0.286
0.363
433.6
0.899
0.105
409.6
0.563
0.161
0.023
526.3
0.123
0.984
512.5
0.085
0.25
489.4
0.268
0.586
458.7
0.526
0.212
436.0
0.127
0.022
428.4
0.252
0.044
412.2
0.777
0.02
535.4
0.149
0.989
551.0
0.085
0.189
578.3
0.253
0.506
624.6
0.441
0.118
683.0
0.169
0.013
750.2
1.246
0.006
533.7
0.147
0.990
549.4
0.082
0.180
576.1
0.250
0.497
622.2
0.420
0.111
682.7
0.233
0.012
750.2
1.246
0.006
R (silica gel, chloroform/ethanol, 100:1) = 0.10. IR (KBr): ν =
˜
f
3435.8 (s), 2926.5 (m), 2856.9 (w), 1698.5 (s), 1660.9 (s), 1594.0 (s),
1578.3 (m), 1507.3 (w), 1458.4 (w), 1432.7 (w), 1405.0 (m), 1347.1
(m), 1329.4 (s), 1253.1 (m), 1175.3 (w), 1108.2 (w), 963.6 (w), 854.3
(w), 840.2 (w), 811.8 (w), 749.0 (w), 722.6 (w), 672.0 (w), 585.8
(w) cm–1. 1H NMR (600 MHz, CDCl3, 25 °C): δ = 0.83 [t, 3J(H,H)
= 7.1 Hz, 6 H, 2 CH3], 1.23–1.34 (m, 16 H, CH2), 1.85–1.89 (m, 2
H, β-CH2), 2.07 (s, 6 H, 2 CH3), 2.23–2.28 (m, 8 H, β-CH2/2 CH3),
5.17–5.22 (m, 1 H, α-CH), 8.67–8.77 (m, 8 H, arom. CH) ppm. 13
C
NMR (151 MHz, CDCl3, 25 °C): δ = 14.0, 14.2, 15.1, 22.6, 26.9,
29.2, 31.8, 32.4, 54.8, 123.1, 123.3, 123.4, 126.5, 126.8, 129.6, 130.1,
131.2, 132.0, 134.5, 135.1, 163.4, 163.6, 164.6 ppm. UV/Vis
(CHCl3): λmax (Erel) = 459 (0.23), 491 (0.61), 527 (1.00) nm. Fluo-
rescence (CHCl3): λmax = 536, 578 nm. Fluorescence quantum yield
(CHCl3, λex = 489 nm, E489nm = 0.277 cm–1, reference: 2a with Φ
= 1.00): 0.53. MS (DEI+/70 eV): m/z (%) = 719 (100) [M]+, 538
(19), 537 (18), 505 (7), 391 (8), 147 (26). HRMS: calcd. for
C47H49N3O4 719.3723; found 719.3653 (∆ = –7.0 mmu).
N2,N3-Bis(1-hexylheptyl)-N1-[N-(1-hexylheptyl)-NЈ-(2,3,5,6-tetra-
methylphenyl-4-yl)perylene-3,4:9,10-tetracarboxylic Bisimide]ben-
zo[ghi]perylene-2,3:8,9:11,12-hexacarboxylic Trisimide (6): Com-
pound 4 (264 mg, 367 µmol), N1,N2-bis(1-hexylheptyl)benzo[ghi]-
perylene-2,3,8,9,11,12-hexacarboxylic-2,3;8,9-bisimide-11,12-anhy-
dride[3] (5, 311 mg, 367 µmol), and imidazole (15 g) were allowed
to react as was described for 4 and purified by column separation
(silica gel 60), where byproducts were firstly eluted with chloroform
and bichromophore 6 collected with chloroform/methanol (25:1).
Yield: 85 mg (15%), bright-red powder. M.p. Ͼ300 °C. Rf (CHCl3/
0.017
0.033
0.016
[a] Calculated wavelength in nm. [b] Linewidth in 106 cm–2 (kK2).
[c] Calculated absorptivity for Emax = 1.00. [d] R = Residual, see
equation below.
Conclusions
methanol, 25:1) = 0.8. IR: ν = 3077.6 (w), 2951.6 (s), 2923.1 (s),
˜
2854.3, (s), 1774.3 (w), 1701.7 (s), 1659.8 (s), 1626.4 (w), 1593.1 (s),
1579.3 (m), 1521.9 (w), 1506.5 (w), 1456.8 (m), 1432.5 (w), 1415.4
(w), 1404.3 (s), 1363.3 (s), 1335.7 (s), 1316.5 (s), 1272.2 (w), 1250.0
(m), 1203.1 (w), 1173.8 (m), 1122.5 (w), 1016.3 (w), 962.8 (w), 944.8
(w), 850.6 (m), 808.4 (s), 767.4 (w), 745.1 (s), 723.9 (w), 660.4 (w),
644.0 (w), 584.4 (w) cm–1. 1H NMR (600 MHz, CDCl3, 25.0 °C): δ
These results are important for the design of molecular
optical devices, because the handling of the energy of op-
tical excitation will become one central problem for such a
technology.
Dye 6 is of interest for many other applications, because
of the broad light absorption in the visible region and the
high fluorescence quantum yield even in the presence of
atmospheric oxygen. Such a combination of properties is
3
3
= 10.6 (br. d, J = 20.4 Hz, 2 H), 9.47 (d, J = 8.5 Hz, 2 H), 9.22
3
3
3
(br. d, J = 15.6 Hz, 2 H), 8.83 (d, J = 7.8 Hz, 2 H), 8.73 (d, J =
8.1 Hz, 2 H), 8.70 (br. d, 3J = 6.8 Hz, 4 H), 5.36–5.27 (m, 2 H),
both of importance for the application as laser dyes and for 5.19 (tt, J = 5.8 and 9.4 Hz, 1 H), 2.40–2.30 (m, 4 H), 2.29 (s, 6
3
fluorescent planar solar collectors[13] and may even be use-
H), 2.28–2.22 (m, 2 H), 2.20 (s, 6 H), 1.98–1.91 (m, 4 H), 1.90–1.83
3
ful for the calibration of fluorescence spectrometers.[14]
(m, 2 H), 1.44–1.20 (div. m, 48 H), 0.83 (t, J = 6.9 Hz, 6 H), 0.82
(t, 3J = 6.9 Hz, 12 H) ppm. 13C NMR (150 MHz, CDCl3, 25.0 °C):
δ = 167.1, 162.8, 135.4, 135.0, 134.4, 134.1, 133.5, 132.9, 132.1,
130.3, 130.2, 129.6, 128.4, 127.8, 127.5, 126.9, 126.5, 125.4, 124.2,
123.6, 123.4, 123.2, 55.3, 54.8, 32.4, 31.8, 31.8, 29.7, 29.2, 29.2,
Experimental Section
General: IR spectra were recorded with a Perkin–Elmer 1420 Ratio
Recording Infrared Spectrometer, FT 1000. UV/Vis spectra were
measure with a Varian Cary 5000 and Bruins Omega 20. Fluores-
cence spectra were acquired with a Perkin–Elmer FS 3000 (totally
corrected). NMR spectra were recorded with a Varian Vnmrs 600
(600 MHz). Mass spectrometry was performed with a Finnigan
MAT 95.
27.0, 26.9, 22.6, 15.9, 15.4, 14.0 ppm. UV/Vis (CHCl3): λmax (Erel):
527 (100), 491 (61), 466 (82), 436 (47), 410 (19), 374 (46) nm. Fluo-
rescence (CHCl3): λmax (Irel) = 535 (100), 577 (36) nm. Fluorescence
quantum yield (CHCl3, λexcit. = 435 nm, E = 0.138 cm–1, ref.[5] per-
ylene-3,4,9,10-tetracarboxylic tetramethyl ester with Φ = 100%):
100%. MS (DEP/EI): m/z (%) = 1550 (100) [M]+. MS (FAB+):
calcd. for C101H108N5O10 1550.8096; found 1550.8094 (∆
=
–0.2 mmu). C101H107N5O10 (1551.0): calcd. C 78.21, H 6.95, N
4.52; found C 77.40, H 7.26, N 4.67.
N-(4-Amino-2,3,5,6-tetramethylphenyl)-N-(1-hexylheptyl)perylene-
3,4:9,10-tetracarboxylic Bisimide (4): Perylene-3,4,9,10-tetracarbox-
ylic-3,4-anhydride-9,10-(1-hexylheptylimide)[8]
(3,
300 mg,
Synthesis of 6 by Microwave Irradiation: Compound 4 (150 mg,
208 µmol), 5[3] (100 mg, 118 µmol), and quinoline (6 mL) were
heated with stirring at 210 °C for 5 h by microwaves (200 W Dis-
523 µmol), 2,3,5,6-tetramethyl-1,4-phenylene diamine (129 mg,
785 µmol), and imidazole (30 g) with the exclusion of moisture and
Eur. J. Org. Chem. 2008, 4559–4562
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H. Langhals, S. Poxleitner, O. Krotz, T. Pust, A. Walter
SHORT COMMUNICATION
phoric Silicones: Ordering Principles in Complex Molecules”
in Silicon Based Polymers (Eds.: F. Ganachaud, S. Boileau, B.
Boury), Springer, 2008, pp. 51–63; ISBN 978-1-4020-8527-7, e-
ISBN 978-1-4020-8528-4.
coverer from CEM) and isolated as was described before. Yield:
65 mg (36%). For spectroscopic data see above.
[5] L. B.-Å. Johansson, H. Langhals, Spectrochim. Acta 1991, 47A,
Acknowledgments
857–861.
[6] a) H. Langhals, R. Kollefrath, J. Lindner, Macromol. Rep.
1995, A32, 415–423; b) H. Langhals, J. Gold, J. Prakt. Chem.
1996, 338, 654–659; c) H. Langhals, W. Jona, Angew. Chem.
1998, 110, 998–1001; Angew. Chem. Int. Ed. 1998, 37, 952–955.
[7] H. Langhals, M. Rauscher, J. Strübe, D. Kuck, J. Org. Chem.
2008, 73, 1113–1116.
We thank SABIC Innovative Plastics for financial support.
[1] a) T. Foerster, Naturwiss. 1946, 33, 166–175; Chem. Abstr. 1947,
41, 36668; b) T. Foerster, Ann. Phys. 6. Folge 1948, 2, 55–75;
Chem. Abstr. 1949, 43, 31172; c) T. Foerster, Z. Elektrochem.
1949, 53, 93–99; Chem. Abstr. 1949, 43, 33629; d) T. Foerster,
Z. Naturforsch. 1949, 4a, 321–327; Chem. Abstr. 1950, 44,
43074.
[8] H. Kaiser, J. Lindner, H. Langhals, Chem. Ber. 1991, 124, 529–
535.
[9] a) S. Demmig, H. Langhals, Chem. Ber. 1988, 121, 225–230; b)
H. Langhals, S. Demmig, T. Potrawa, J. Prakt. Chem. 1991,
333, 733–748.
[2] a) S. W. Hell, L. Kastrup, Nachr. Chem. Technol. 2007, 55, 47–
50; Chem. Abstr. 2007, 146, 38018; b) H.-P. Braun, U. K. [10] H. Langhals, S. Demmig, H. Huber, Spectrochim. Acta 1988,
Schmitz, Ann. Plant Rev. 2006, 28, 55–70; Chem. Abstr. 2006,
146, 311667; c) F. D. Lewis, Pure Appl. Chem. 2006, 78, 2287–
2295; d) I. L. Medintz, Trends Biotechnol. 2006, 24, 539–542;
Chem. Abstr. 2006, 146, 117068; e) A. Periasamy, R. N. Day, J.
Biomed. Opt. 2006, 11, 069901/1–069901/2; Chem. Abstr. 2007,
147, 90184; f) K. E. Sapsford, L. Berti, I. L. Medintz, Angew.
Chem. Int. Ed. 2006, 45, 4562–4588; g) S. Nagl, M. Schaefer-
ling, O. S. Wolfbeis, Microchim. Acta 2005, 151, 1–21; Chem.
Abstr. 2005, 144, 66072.
44A, 1189–1193.
[11] a) Calculation code of MNDO: M. J. S. Dewar, W. Thiel, J.
Am. Chem. Soc. 1977, 99, 4899–4907; b) J. J. P. Stewart, Param-
etrization of MNDO (AM1): Program MOPAC (version 6.0),
program parameter: PRECISE, XYZ.
[12] a) H. Langhals, Spectrochim. Acta Part A 2000, 56, 2207–2210;
b) H. Langhals, Anal. Bioanal. Chem. 2002, 374, 573–578.
[13] H. Langhals, Nachr. Chem. Tech. Lab. 1980, 28, 716–718,
Chem. Abstr. 1981, 95, R9816q.
[3] H. Langhals, S. Kirner, Eur. J. Org. Chem. 2000, 365–380.
[4] a) Review: H. Langhals, Helv. Chim. Acta 2005, 88, 1309–1343;
b) Review: H. Langhals, Heterocycles 1995, 40, 477–500; c) Re-
view: H. Langhals, “Molecular Devices. Chiral, Bichromo-
[14] S. Kalinin, M. Speckbacher, H. Langhals, L. B.-Å. Johansson,
Phys. Chem. Chem. Phys. 2001, 3, 172–174.
Received: May 6, 2008
Published Online: August 15, 2008
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