Facile pathways to multichromophoric arrays based on a truxene platform

Tetrahedron Letters 50 (2009) 1203–1208

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Facile pathways to multichromophoric arrays based on a truxene platform Stéphane Diring, Raymond Ziessel * Laboratoire de Chimie Organique et Spectroscopies Avancées (LCOSA), ECPM, UMR 7515 25 rue Becquerel 67087 Strasbourg Cedex, France

a r t i c l e

i n f o

Article history: Received 8 December 2008 Accepted 23 December 2008 Available online 7 January 2009

a b s t r a c t Multi-branched dyes were synthesized using a truxene platform appropriately functionalized with iodo residues, the key step here being a controlled iodination process giving access to mono-, di-, tri- and hexa-derivatives. Conversion of the iodo compounds to the corresponding alkynes followed by cross-coupling with iodophenylindacenes catalysed by Pd(0) or with chloro-Pt(II) complexes catalysed by Cu(I) enabled the introduction of multiple chromophore/luminophore units. Both neutral and cationic species involving Pt(II) centres were obtained in excellent yields. Connection of ethynylpyrene units to boron within the hexa-grafted truxene Bodipys provided very efficient cascade singlet energy transfer from the pyrene to the yellow emitter. Irradiation in the truxene core also resulted in quantitative energy transfer to the organic dyes. Ó 2009 Elsevier Ltd. All rights reserved.

In recent years, the engineering of artificial systems devoted to collection and concentration of photons has attracted significant attention.1,2 As a rule, such systems contain a molecular platform consisting of luminescent modules (transition metal complexes, organic dyes or luminescent polymers) connected via unsaturated bridges. The concept is to absorb light at several equivalent sites and transport all photons to a single, different emitting or reacting site. Provided energy transfer is rapid relative to deactivation, the result is concentration of the incident light energy. The same principle applies to photosynthetic organisms that use light-harvesting centres to optimize absorption of solar energy, and then transfer this excitonic energy to the reaction centre, where it is used to bring about reactions such as oxidation of water and reduction of carbon dioxide.3 Nature has ideally organized the chromophores, and relays in such a way that each step of the collection process is very fast (subpicosecond regime). Chemists have learnt how to carefully design artificial systems mimicking some of these fundamental processes, and many have found application in energy conversion devices,4 redox switches, photonic wires and gates,5 and logic systems.6 Truxene (10,15-dihydro-5H-diindeno[1,2-a;10 ,20 -c]-fluorene) is a planar heptacyclic aromatic system,7 which can be formally regarded as a C3-symmetrically fused fluorene trimer. During the past decades, it had been recognized as potential starting material for the construction of liquid crystalline compounds,8 bowl-shaped fragments of fullerenes9 and C3-tripodal materials for chiral recognition.10 Recently synthesized derivatives include a variety of starshaped oligomers and dendritic truxene derivatives with extended p-conjugation11 of the polyaromatic core, as well as truxene-based

* Corresponding author. Tel.: +33 3 90 24 26 89; fax: +33 3 90 24 27 61. E-mail address: [email protected] (R. Ziessel). 0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2008.12.100

donor–acceptor systems for use as multifunctional fluorescent probes.12 In this contribution we report the synthesis, characterization and some optical properties of a series of novel truxene derivatives bearing one to six chromophoric units appended to the central truxene core via ethynyl linkers. Highly fluorescent difluorobora-diaza-s-indacene (F-Bodipy) derivatives or polyimine Pt(II) fragments were connected as branches to the truxene moiety via metal-catalyzed cross-coupling reactions. The target ethynyltruxene derivatives A1–A6 were synthesized starting from the readily available hexabutylated truxene 1, which was obtained by peralkylation of the anions of the truxene lithium salts.13 Although the selective mono- and di-iodination of truxene derivatives have been described in the literature,14 in our case the reactions always provided a mixture of mono- and di-iodinated compounds which could not be purified by column chromatography, and necessitated tedious and repetitive recrystallizations. Thus, the mixture of mono and diiodinated compounds I1 and I2 was directly reacted with propargyl alcohol via palladium-catalyzed cross-coupling reaction. The polarity brought by the alcohol functions allowed direct separation of the mono- and bis-propargylic substituted derivatives by column chromatography. The corresponding ethynyl derivatives A1 and A2 were obtained by treatment with manganese oxide and potassium hydroxide. Triiodinated truxene I3 was obtained by treatment with a mixture of iodine and iodic acid, while increasing the reaction temperature as well as the iodine/iodic acid amount led to the formation of the hexasubstituted compound I6. Reaction of I3 and I6 with (trimethylsilyl)acetylene using Pd(0) catalysis and subsequent treatment with either potassium fluoride or potassium carbonate afforded the corresponding ethynyl derivatives A3 and A6 in good yields (see Scheme 1).

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S. Diring, R. Ziessel / Tetrahedron Letters 50 (2009) 1203–1208 1

2

C4H9

H H

13

14

3 4

H

11

C4H9

H

H 9

6

1

8

7

b)

b) I

I

C4H9

C4H9 C4H9

C4H9

H

I

C4H9

a)

12

C4H9

C4H9

c)

C4H9

C4H9

I

C4H9

I C4H9

C4H9

C4H9 C4H9

C4H9

I2

I1 d,e)

C4H9

C4H9

C4H9

C4H9

C4H9 C4H9

C4H9

I

f,g)

f,h)

C4H9

C4H9

C4H9

C4H9

A2

A1

i)

C4H9 C4H9

C4H9

A3

i)

A6

i)

F F B N N

C4H9

C4H9

C4H9 C4H9

C4H9

I6

I

I

C4H9

C4H9 C4H9

C4H9

C4H9 C4H9

C4H9

I3

d,e)

C4H9

I C4H9

C4H9 C4H9

C4H9

C4H9 C4H9

C4H9

I

C4H9

I I

+

C4H9

C4H9

i)

F F B N N

I

N N B F F C4H9

C4H9

C4H9

C4H9

C4H9

C4H9 C4H9

C4H9

C4H9

B1

N F B F N

C4H9

2

C4H9 C4H9

B3

F F B N N

F F B N N

N

B N F F

C4H9

F F N B N

C4H9

C4H9

N B F N

N F B F N

C4H9

F

C4H9 C4H9

N F

B N F

C4H9

C4H9 C4H9

B2

C4H9

N F

B N F

C4H9 C4H9

N N B F F

B6

Scheme 1. Reagents and conditions: (a) n-BuLi, THF, n-C4H9Br, 0 °C to rt; (b) I2/HIO3, AcOH, H2SO4, H2O, CCl4, 80 °C, 3 d; (c) I2/HIO3, AcOH, H2SO4, H2O, CCl4, 115 °C, 18 h; (d) propargyl alcohol, [Pd(PPh3)4], n-propylamine, 65 °C, 18 h; (e) MnO2, KOH, Et2O, rt, 18 h; (f) (CH3)3SiC„CH, [Pd(PPh3)2Cl2] CuI, THF, iPr2NH, rt, 18 h; (g) K2CO3, CH2Cl2, CH3OH, rt, 2 h; (h) KF, THF, CH3OH, rt, 18 h; (i) [Pd(PPh3)4], C6H6, Et3N, 65 °C, 18 h.

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S. Diring, R. Ziessel / Tetrahedron Letters 50 (2009) 1203–1208 Table 1 Selected data for compounds A1–A6, B1–B6 and P12 Compound

Yield (%)

MSa

k (nm), e (M1cm1)b

kem (nm)b

c em

s (ns)

kr d(108 s1)

knr d(108 s1)

A1 A2 A3 A6 B1 B2 B3 B6 P12

66 70 92 93 74 79 78 84 72

702.4 727.0 750.3 822.4 1080.5 1484.0 1887.1 3094.0 5567.6

325 325 326 334 527 527 526 526 523

361, 365, 369, 370, 540 540 540 542 538

0.20 0.21 0.23 0.22 0.80 0.75 0.57 0.72 0.80

17.5 18.2 20.3 18.3 5.0 5.4 5.3 5.2 5.5

0.11 0.12 0.11 0.12 1.60 1.39 1.08 1.38 1.45

0.457 0.434 0.379 0.426 0.40 0.46 0.81 0.54 0.36

(40900) (92300) (159700) (95800) (77900) 333 (59900) (149300) 337 (117700) (189000) 339 (152900) (407000) 351 (182200) (438000) 371 (799000)

378 383 387 389

a

Electron impact mass spectrometry, referring to [M]+, for P12 MALDI-TOF in dichloromethane solution. Determined in aerated dichloromethane solution, ca. 5  106 M. c Quantum yield of emission, calculated using quinine bisulfate (Uem = 0.55 in H2SO4 1 N, kexc = 366 nm) as reference for compounds A1–A6 and rhodamine 6G (Uem = 0.78 in water, kexc = 500 nm),15 as reference for compounds B1–B6 and P12, all Uem are corrected for changes in refractive index. d Radiative and non-radiative rate constants kr and knr calculated using the following equations: kr = UF/sF, knr = (1  UF)/sF. b

C12H25O

OC12H25

C12H25O H

C12H25O

N

C12H25O C4H9

C4H9 C4H9

C4H9

C12H25O

N Pt Cl

N

N Pt

3

C4H9 C4H9 C4H9

C4H9

C4H9

A1

C1

C4H9 C4H9

C4H9

Scheme 2. Reagents and conditions: CuI, CH2Cl2, Et3N, rt, 72 h, 82%, anaerobic conditions.

After some experimentation, we succeeded in preparing compounds B1, B2, B3 and B6 bearing one, two, three and six Bodipy fragments, respectively, by reacting stoichiometric amounts of the ethynyl-truxene derivatives with the iodophenyl-Bodipy derivative 216 under classical Pd(0) cross-coupling conditions. No decrease in yields was observed in increasing the number of Bodipy linkages (see Table 1). In an effort to incorporate other chromophores and in analogy with the previous reactions, we decided to functionalize the truxene core with luminescent oligopyridine d8-Pt(II) complexes (Scheme 2). To avoid possible solubility problems associated with unsubstituted (phenylpolypyridine)Pt(II) units, we used orthometallated (6-phenyl-2,20 -bipyridine)Pt(II) precursors bearing the tridodecyloxyphenyl moiety.17 The connection to the truxene core was promoted by CuI leading to the neutral complex C1 in 82% yield. Analogous (tert-butyl-terpyridyl)Pt(II) truxene derivatives were obtained from (4,40 ,400 -tert-butyl-2,20 :60 ,200 -terpyridine)Pt(II) chloride under similar conditions.18 Neutral or cationic mono-, diand tri-nuclear complexes were obtained in good yields (Chart 1). The functionalization opportunities offered by the Bodipy family of compounds enabled us to extend the light-harvesting abilities of the hexasubstituted compound B6. Displacement of the fluorine atoms in favour of the formation of a boron-ethynyl bond can be achieved by using a Grignard reagent. Thus, 1-ethynylpyrene was reacted with ethylmagnesium bromide to form the corresponding Grignard derivative, and then allowed to react with compound B6. Complete boron functionalization occurred in good yield (72%), and afforded P12 bearing 12 pyrene fragments on its periphery (see Scheme 3). All compounds were unambiguously characterized by 1H and 13 C NMR, electron impact mass spectrometry and elemental anal-

ysis. In particular, the well-resolved NMR spectra for all compounds excluded the presence of aggregates. 1 H NMR spectra for compounds B1, B6 and P12 are shown in Figure 1. For B1, the characteristic signals corresponding to the truxene alkyl chains as well as to the substituents on the Bodipy core can be clearly identified in the aliphatic region. The aromatic signals are quite complex due to the lack of symmetry in B1, but an AB signal (7.53 ppm, JAB = 8.2 Hz, mdAB = 123.0 Hz) can be safely

C12H25O

OC12H25 OC12H25 +

N Pt

N

N

C C4H9

C4H9 C4H9

C1 D1

Pt

N

D C4H9

C4H9

C4H9 C4H9

BF4-

N

C4H9

C4H9 C4H9

C4H9

C4H9

C4H9

C2 D2 Chart 1.

C4H9 C4H9

C3 D3

C4H9

C4H9 C4H9

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S. Diring, R. Ziessel / Tetrahedron Letters 50 (2009) 1203–1208

FF N BN

BN N

F F BN N

NB N

N F BF N

C4H9 C H 4 9

N B N

i) N FB F N

C4H9 C4H9

C4H9 C4H9

C4H9 C4H9 C4H9 C4H9

72%

N B N

C4H9 C4H9

N N BFF N BN

N N B FF

N NB

P12

B6

Scheme 3. Reagents and conditions: 1-ethynylpyrene, EtMgBr, THF, 60 °C, 18 h.

attributed to the protons on the phenyl ring in the meso position of the Bodipy. When six bodipy fragments are connected to the truxene core (B6), the aromatic pattern becomes much simpler, with two singlets corresponding to the six protons remaining on the truxene moiety and two distinct AB signals referring to the protons

ab and cd of the meso phenyl groups. In the aliphatic region, two singlets at 1.38 and 1.35 ppm accounting for protons 1 and 10 confirm that the Bodipy fragments at the C 2,7,12 positions differ chemically from those at the C 3,8,13 positions. For P12, the presence of the 12 pyrene fragments is consistent with a characteristic

2

3

F F B N N

CDCl3

4

a

2, 6H

b

1, 6H

a,b 4H

1

4, 6H 3H

6H

1H

C4H9

C4H9 C4H9

3, 4H

1H 8.4

7.8

2.8

7.4

2.4

2.0

1.2

0.8

C4H9 C4H9

(B1)

0.4 F F B N N

CDCl3

36H 1 + 1’

24H a,b + c,d

C4H9

1

a

b 1 1' c d

C4H9

F N B F N

Cy

C4H9 C4H9

1'

3HT1 3HT4

C4H9 C4H9

HT4

HT1

8.6

7.6

3.0

1.4

2.5 (ppm)

1.2

1.0

0.8

C4H9

0.6

(B6)

96H + 3HT1 + 12H b, d CDCl3

2

3

pyrene 108H

4

B N N

a

1 b

12H

1'

12H a, c

1' c d

C4H9

N B N

3HT4

1'

HT1

8.8

8.6

8.4

8.2

8.0

7.8

7.6

2 36H + 6H

C4H9 C4H9

HT4

7.4

18H + 18H 1’ + 1

(P12) 4 36H + 12H

H2O

3 24H + 6H

C4H9

C4H9 C4H9

24H + 6H

Et2O

6H 3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8 ppm

1.6

1.4

1.2

1.0

0.8

0.6

Figure 1. Proton NMR (400.1 MHz) spectra measured in CDCl3 at room temperature for key dyes B1 (top), B6 (middle) and P12 (bottom).

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S. Diring, R. Ziessel / Tetrahedron Letters 50 (2009) 1203–1208

900000 800000 B1 B2 B3 B6 P12

ε / mol-1.cm-1

700000 600000 500000 400000 300000 200000 100000 0 260

310

360

410

460

510

560

Absorption Wavelength / nm Figure 2. Absorption spectra recorded for the different truxene-Bodipy dyes in CH2Cl2 solution at rt.

doublet at 8.78 ppm (3J = 9.2 Hz). The aliphatic part is comparable to that in B6 although the alkyl signals of the truxene chains are less well resolved due to the aromatic pyrene envelope. As shown in Figure 2, compounds B1 to B6 present essentially the same absorption profile. The intense absorption band at 526 nm which is approximately proportional to the number of fluorophore branches can be attributed to the S0?S1 transition located on the Bodipy core. The spectral region between 310 and 410 nm shows absorption bands, which progressively increase in intensity from B1 to B6 and are assigned to p–p* excitations of the truxene units and S0?S2 of the Bodipy cores. Note that for P12, the additional strong absorptions located near 270 and 370 nm and displaying well-resolved fine structure are due to transitions belonging to the pyrene subunits. The emission spectra for compounds B1 to B6 are shown in Figure 3. All compounds show a single emission at 540 nm, which is characteristic of Bodipy emission and independent of the excitation wavelength. Excitation in the truxene absorption range of 310–340 nm produced no truxene emission at 362 nm. In a similar way, excitation in the pyrene transitions does not provide any residual pyrene emission but only that from Bodipy. Clearly, quantitative intramolecular energy transfer from the truxene core or pyrene fragments is effective in these multichromophoric systems.

As would be expected from this, the excitation spectra match the absorption spectra over the complete absorption range, which is a clear indication of energy transfer and that all modules participate in the energy-transfer process. Spectroscopic data for all dyes are gathered in Table 1. All the platinum complexes exhibit a strong absorption, around 477 nm for C1–C3 and in the range 496–512 nm for D1–D3, corresponding to a metal-to-ligand charge-transfer transition (MLCT) with a contribution of ligand (alkyne-truxene)-to-ligand (terpy or phenylbipy)-charge transfer (LLCT). The former series phosphoresce at 622 nm and the latter at 649 nm, values in keeping with previous studies on mono-nuclear complexes.19 Our design opens up the possibility of engineering sophisticated dyes bearing photon absorbers at the periphery and at the core, and energy transducers in the inner layer. All photons would be funnelled to a low-energy acceptor (Bodipy) before the final emission. Direct grafting of platinum modules via r-alkyne tethers provides phosphorescent multi-nuclear complexes in which the inherent fluorescence of the truxene is quenched by the MLCT and LLCT states of the complex. Work in progress is focused on increasing the dimensionality of these multibranched systems and on introducing additional energy relays in order to promote multi-FRET energy devices. Such a strategy

5.0E+05

Fluorescence Intensity (a.u.)

4.5E+05 B1 B2 B3 B6 P12

4.0E+05 3.5E+05 3.0E+05 2.5E+05 2.0E+05 1.5E+05 1.0E+05 5.0E+04 0.0E+00 500

520

540

560

580

600

620

640

660

680

Fluorescence Wavelength / (nm) Figure 3. Emission spectra recorded for the different Bodipy dyes linked to truxene in CH2Cl2 solution at rt. Excitation wavelength = 330 nm.

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S. Diring, R. Ziessel / Tetrahedron Letters 50 (2009) 1203–1208

requires distinction of the different reacting sites on the truxene platform. Further work is in progress to obtain multi-cascade molecular devices. References and notes 1. Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57. 2. Harriman, A.; Ziessel, R. In Carbon-Rich Compounds; Haley, M. M., Tykwinski, R. R., Eds.; Wiley-VCH: Weiheim, 2006; pp 26–89. and references cited therein. 3. Blankenship, R. E. In Molecular Mechanism of Photosynthesis; Blackwell Science: Oxford, UK, 2002. 4. Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1996, 118, 11166; Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635. 5. Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759. 6. Hindin, E.; Forties, R. S.; Loewe, R. S.; Ambroise, A.; Kirmaier, C.; Bocian, D. F.; Lindsey, J. S.; Holten, D.; Knox, R. S. J. Phys. Chem. B 2004, 108, 12821. 7. Gomez-Lor, B.; De Frutos, O.; Ceballos, P. A.; Granier, T.; Echavarren, A. M. Eur. J. Org. Chem. 2001, 2107–2114.

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