Synthesis, Characterization and Photophysical Properties of DCM-Based Light-Harvesting Dendrimers

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Synthesis, Characterization and Photophysical

Properties of DCM-Based Light-Harvesting

Dendrimers

Mahalingam Vanjinathan, Hong-Cheu Lin, A. Sultan Nasar*

Introduction

Light-emitting dendrimers are macromolecules with a

well-defined structure and are composed of a core,

dendrons and surface groups. The appropriate selection

of each of these components provides a good and

independent control of the electronic and solution

proces-sing properties. This makes them convenient model

systems to study the organic semiconductor physics.

[1–3]

In contrast to linear polymers, the beauty of dendrimers is

their size and architecture that can be specifically controlled

during the synthesis.

[4]

_{A characteristic of dendritic}

macromolecules is the presence of a single core surrounded

by numerous peripheral chain ends. The globular shape of

dendrimers provides a large surface area that can be

decorated with chromophores, resulting in a large

absorp-tion cross-secabsorp-tion and enabling effective capture of

photons.

[5]

In a dendritic antenna, the peripheral donor

units collect photons and transfer the excitation energy

through space to the core or focal point acceptor

chromophores, i.e., dendrons acts as the ‘‘molecular lens’’.

[6]

For an efficient energy transfer from the outer to the inner

dye arrays, a pair of chromophores is required which should

be capable of exhibiting high fluorescence quantum yields,

and should have a good spectral overlap of the absorption

spectrum of the energy acceptor dye with the fluorescence

band of the energy donor dye. In addition to these two

criteria, individual excitation of specific absorption band in

the dyes should also be considered for an efficient energy

transfer. The first two properties are prerequisite for an

efficient space-through resonance energy transfer (Fo

¨rster

mechanism)

[7]

and the latter condition is important for an

easy evaluation of the energy transfer efficiency.

If we consider a dendrimer containing pyrene units as

surface groups, that groups can act as energy donors (light

antenna) for core acceptor chromophores. The fluorescence

M. Vanjinathan, H.-C. Lin

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan

A. S. Nasar

Department of Polymer Science, University of Madras, Guindy Campus, Chennai-600025, India

E-mail: [email protected]

A series of highly soluble light-harvesting dendrimers up to third generation were synthesized

using a DCM-based core and Frechet-type benzyl ether dendrons containing pyrene end

groups. All dendrimers and intermediates were characterized by FTIR,

1

H NMR, MALDI-TOF

MS, UV-Vis, and PL spectra and EA, GPC, and TGA techniques. The dendrimers emit strong red

PL around 570 nm. The excitation of the terminal chromophores results in core emission alone,

as the donor emission is seriously quenched due to FRET to the core. The dendrimers showed

enhanced luminescence properties of

the core, and its efficiency was

depen-dent on the generation number of the

dendrimers. The above results reveal

that these dendrimers are suitable

can-didates for red emissive materials in

OLEDs.

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properties of pyrene are well known and characterized by

long excited-state lifetimes

[8]

_{and distinct solvatochromic}

shifts.

[9]

Furthermore, pyrene exhibits characteristic

exci-mer formation in concentrated solutions and in the solid

state, due to self-association of the polyaromatic

hydro-carbon moieties. However, this leads to a dramatic decrease

in fluorescence and also to less defined, broadened

fluores-cence spectra. Therefore, excimer formation of pyrene can be

used to study aggregation phenomena.

[8b,10]

Moreover,

monolayers and thin films containing pyrene derivatives

turned out to be promising candidates for several

applica-tions such as organic light-emitting diodes (OLEDs);

[11]

light-emitting materials have been reviewed very recently by Li

and Bo.

[12]

_{Thayumanavan and co-workers studied the photo}

physics of a series of non-conjugated systems consisting of a

benzthiadiazole core and benzyl ether arms terminated with

aminopyrene chromophores, which function as both energy

and electron donors.

[13]

2,6-Dimethyl-4H-pyran-4-ylidene)malononitrile (DCM)

is a fluorescent material with good chemical stability and

its derivatives are known to be used as promising

low-molecular-weight red-emitting materials.

[14–18]

All

DCM-class red dyes contain the 2-pyran-4-ylidenemalononitrile

(PM) moiety as electron acceptor. The synthesis and

characterization of numerous DCM-type molecules and

their applications in electroluminescent devices were

reviewed by Chen.

[19]

New derivatives of DCM for device

applications were published by Jung et al.

[20]

Jin and

co-workers reported the synthesis and luminescent properties

of fluorene copolymers bearing DCM pendants.

[21]

_Recently,

Choi and co-workers reported the red-emitting

phenothia-zine dendrimers encapsulated with DCM derivatives.

[22]

So far, light harvesting molecule of pyrene/DCM

combination has not been reported. In the present work,

for the first time, we report the synthesis and photophysical

properties of dendrimers up to third generation consisting

of pyrene as energy donor, DCM as red-emitting and energy

acceptor core and non-conjugated benzyl ether branches.

We found that the electron-rich pyrene molecules are

excellent light-harvesting molecular wedges whose

photo-luminescence (PL) emission was well suited to excite the

red-emitting core.

Experimental Part

Chemicals

4-Dicyanomethylene-2,6-dimethyl-4H-pyran (8) was synthesized from 2,6-dimethyl-4-pyrone as described by Woods.[23]

Pyrene-1-carboxaldehye (Lancaster), 3,5-dihydroxybenzoic acid (Lancaster), N,N-diisopropylazido dicarboxylate (DIAD, TCI Chemicals Co.), triphenylphosphine (PPh3, Lancaster), LiAlH4(Lancaster), piperidine

(Across Chemicals Co.), malononitrile (Lancaster), acetic anhydride (Lancaster), 4-hydroxy-3,5-dimethoxybenzaldehyde (Lancaster), and trichloroethane (Merck) were used without further purification.

Tetrahydrofuran (THF) was distilled to keep anhydrous before use. Other solvents were purified by standard procedures.

Measurements

Fourier-transform infrared (FTIR) spectra of samples (dispersed in KBr discs) were recorded on a Perkin-Elmer Spectrum 100 Series instrument.1_{H NMR spectra were recorded on a Varian Unity}

300 MHz spectrometer using CDCl3and DMSO-d6as solvents. The

electron impact (EI) mass spectra were recorded using a JEOL DX-303 spectrometer. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Thermogravimetric analyses (TGA) were carried out on a TA Instruments Q500 thermogravimetric analyzer at a heating rate of 20 8C min1under nitrogen. Gel permeation chromatography (GPC) measurements were made on a Waters liquid chromatograph equipped with a 410 differential refractometer (refractive-index detector). THF contain-ing 0.01% lithium bromide was used as an eluent at a flow rate of 1 mL min1. Styragel columns of pore size 103, 104, 105and 106A˚ were used. The molecular weight calibrations were carried out using polystyrene standards having molecular weight (Mw) in the range of 2.9 103 _{to 1.7 10}5 _{g mol}1_{. Matrix-assisted laser}

desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed on a Micromass Tof Spec 2E instrument using a nitrogen 337 nm laser (4 ns pulse) and 2,5-dihydroxyben-zoic acid as a matrix. UV-Vis absorption spectra and PL spectra were recorded on an HP G1103A spectrophotometer, and Hitachi F-4500 spectrophotometer, respectively, in dilute THF solutions (106M).

Synthesis of Pyrene-2ylmethanol (1)

5.0 g (21.7 mmol) of pyrene-1-carboxaldehye was dissolved in 150 mL of dry methanol and the solution was cooled under ice. Under vigorous stirring, 4.2 g (108.66 mmol) of NaBH4was added

portionwise. After completion of the addition, the temperature was increased to room temperature and maintained for 3 h. Excess reducing agent was cautiously destroyed by drop wise addition of 15 mL of dilute HCl at 0 8C. Then, the solvents were removed under vacuum, and the residue was extracted three times with 300 mL of CH2Cl2. The solvent was removed under vacuum after drying with

Na2SO4. Purification of the crude product by column

chromato-graphy on silica gel using 4:1 mixture of hexane and dichlor-omethane gave a light green coloured compound. Yield 4.83 g (96%). IR (KBr): n ¼ 3 562, 3 065, 3 035, 2 929, 2 870, 1 590, 1 458, 1 376, 1 169, 1 067 cm1.1H NMR (CDCl3, 300 MHz): d ¼ 7.99–8.39 (m, 9H,

pyrene H), 5.41 (s, 2H, pyrene-CH2). MS (70 eV, EI): m/z ¼ 232.1

[Mþ]. C17H12O: Calcd. C 87.90, H 5.21, O 6.89; Found C 87.68, H 5.19.

Synthesis of

2-(2,6-Bis[2-{(4-hydroxy-3,5-

dimethoxy)phenyl}vinyl]-4H-pyran-4-ylidene)malononitrile (9)

1.72 g of compound 8 (9.99 mmol), 4.0 g of 4-hydroxy-3,5-dimethoxybenzaldehyde (22 mmol) and five drops of piperidine were dissolved in 150 mL of dry acetonitrile at 90 8C and stirred for overnight under reflux temperature. The reaction mixture was cooled; the solution was filtered through a D3 glass frit to isolate the product. After washing the product twice with 20 mL of methanol,

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thin-layer chromatogram confirmed absence of impurities in the product. The product was dried in vacuo. Recrystallization of this product in methylene chloride gave the product (9) as light yellow crystalline solid. Yield 4.30 g (86%).

IR (KBr): n ¼ 3 510, 3 068, 2 932, 2 223, 1 654, 945 cm1.1H NMR (DMSO-d6, 300 MHz): d ¼ 3.83 (s, 12H), 6.71 (s, 2H), 7.14 (s, 4H), 7.29

(d, 2H), 7.69 (d, 2H), 9.10 (br, 2H, phenolic OH). MS (70 eV, EI): m/z ¼ 500.1 [Mþ]. C28H24N2O7: Calcd. C 67.19, H 4.83, O 22.38,

N 5.60; Found C 66.89, H 4.67, N 5.45.

General Procedure for Synthesis of Dendrons (2), (4),

(6), Model Compound (13) and G1, G2, G3 Dendrimers

Using Mitsunobu Conditions

A mixture of appropriate pyrene-substituted benzyl alcohol (2.2 equiv.), 3,5-dihydroxymethyl benzoate (1.0 equiv.), triphenylpho-sphine (2.2 equiv.) and DIAD (2.2 equiv.) in 100 mL dry THF was kept under sonication for 2.0 h at room temperature followed by stirring at room temperature under nitrogen atmosphere for 24 h. The reaction mixture was evaporated to dryness, and portioned between CH2Cl2and water. The aqueous layer was extracted with

CH2Cl2(3 100 mL), the combined organic layers were dried and

evaporated to dryness. The crude product was purified as outlined in the following.

Synthesis of Pyrene G1-COOMe Dendron (2)

The dendron (2) was prepared from pyrene-2yl-methanol (1) and 3,5-dihydroxymethyl benzoate. Purification of the crude com-pound by column chromatography on silica gel (CH2Cl2/

Et2O) ¼ 50:1) gave (2) as a white solid. Yield 73%.

IR (KBr): n ¼ 3 089, 3 062, 3 031, 2 930, 2 873, 1 720, 1 595, 1 448, 1 372, 1 156, 1 046 cm1_.1_{H NMR (CDCl}

3, 300 MHz): d ¼ 3.88 (s, 3H),

5.30 (s, 4H), 7.00 (t, 1H), 7.49 (d, 2H), 8.00–8.26 (m, 18H, pyrene H). MS (70 ev, EI): m/z ¼ 596 [Mþ]. C42H28O4: Calcd. C 84.54, H 4.73, O 11.53;

Found C 84.23, H 4.45.

Synthesis of Pyrene G2-COOMe Dendron (4)

The dendron (4) was prepared from pyrene G1-CH2OH (3) and

3,5-dihydroxymethyl benzoate. Purification of the crude compound by column chromatography eluting with CH2Cl2/hexane ¼ 70:30 gave

(4) as a light yellow solid. Yield 68%.

IR (KBr): n ¼ 3 075, 3 058, 3 028, 2 938, 2 865, 1 728, 1 586, 1 459, 1 364, 1 165, 1 058 cm1_.1_{H NMR (DMSO-d} 6, 300 MHz): d ¼ 3.78 (s, 3H), 5.00 (s, 8H), 5.52 (s, 4H), 6.05 (t, 1H), 6.17 (d, 4H), 6.45 (t, 2H), 6.82 (d, 2H), 8.03–8.40 (m, 36H, pyrene H). MALDI-TOF MS m/z ¼ 1 268 [Mþ]. C90H60O8: Calcd. C 85.15, H 4.76, O 10.08; Found C 84.98, H 4.64.

Synthesis of Pyrene G3-COOMe Dendron (6)

The dendron (6) was prepared from pyrene G1-CH2OH (5) and

3,5-dihydroxymethyl benzoate. Purification of the crude compound by column chromatography eluting with CH2Cl2/hexane/

THF ¼ 80:10:1 gave (6) as a white solid. Yield 59%.

IR (KBr): n ¼ 3 068, 3 055, 3 020, 2 935, 2 869, 1 720, 1 575, 1 465, 1 356, 1 159, 1 063 cm1.1H NMR (DMSO-d6, 300 MHz): d ¼ 3.70 (s, 3H), 4.31 (s, 8H), 5.00 (s, 4H), 5.58 (s, 16H), 6.07 (t, 6H), 6.19 (d, 12H), 6.48 (t, 1H), 6.89 (d, 2H), 8.01–8.38 (m, 72H, pyrene H). MALDI-TOF MS m/z ¼ 2 612 [Mþ]. C186H124O6: Calcd. C 85.43, H 4.78, O 9.79; Found C 85.20, H 4.56.

Synthesis of G1 Dendrimer (10)

This was prepared from pyrene-G1-CH2OH (3) and compound (9).

Purification of the crude compound by column chromatography eluting with CH2Cl2/hexane/THF ¼ 60:35:5 gave G1-dendrimer

(10) as a light brown coloured solid. Yield 57%.

IR (KBr): n ¼ 3 068, 2 933, 2 265, 1 676, 1 467, 1 365, 1 160, 1 067, 976 cm1_. 1_{H NMR (DMSO-d} 6, 300 MHz): d ¼ 3.81 (s, 12H), 4.30 (s, 2H), 5.01 (s, 8H), 6.06 (t, 2H), 6.18 (d, 4H), 6.72 (s, 2H), 7.15 (s, 4H), 7.26 (d, 2H), 7.68 (d, 2H), 8.09–8.38 (m, 36H, pyrene H). MALDI-TOF MS m/z ¼ 1 602 [Mþ_{þ 1]. C} 110H76N2O11: Calcd. C 82.48, H 4.79, N 1.75; Found C 82.00, H 4.56, N 1.68.

Synthesis of G2 Dendrimer (11)

IR (KBr): n ¼ 3 064, 2 932, 2 230, 1 654, 1 478, 1 365, 1 167, 1 055, 956 cm1.1H NMR (DMSO-d6, 300 MHz): d ¼ 3.86 (s, 12H), 4.35 (s, 8H), 5.09 (s, 4H), 5.25 (s, 16H), 6.11 (s, 2H), 6.16 (s, 4H), 6.22 (s, 4H), 6.33 (s, 8H), 6.65 (s, 2H), 7.15 (s, 4H), 7.21 (d, 2H), 7.65 (d, 2H), 8.01– 8.37 (m, 72H, pyrene H). MALDI-TOF MS m/z ¼ 2 949 [Mþþ 2]. C206H140N2O19: Calcd. C 83.95, H 4.79, N 0.95; Found C 83.45, H 4.57, N 0.91.

Synthesis of G3 Dendrimer (12)

IR (KBr): n ¼ 3 058, 2 928, 2 230, 1 658, 1 472, 1 354, 1 166, 1 062, 958 cm1.1H NMR (DMSO-d6, 300 MHz): d ¼ 3.86 (s, 12H), 4.33 (s, 4H), 5.06 (s, 8H), 5.23 (s, 16H), 5.56 (s, 32H), 6.09–6.60 (m, 42H), 6.67 (s, 2H), 7.18 (s, 4H), 7.23 (d, 2H), 7.66 (d, 2H), 8.03–8.35 (m, 144 H, pyrene H). MALDI-TOF MS m/z ¼ 5 639 [Mþ_{þ 2]. C} 398H268N2O35: Calcd. C 84.78, H 4.79, N 0.50; Found C 84.19, H 4.67, N 0.48.

Synthesis of Model Compound (13)

This was prepared from pyrene-2yl-methanol (1) and compound (9). Purification of the crude compound by column chromatography eluting with CH2Cl2/hexane ¼ 50:10 gave model compound (13) as

a light brown coloured solid. Yield 62%.

IR (KBr): n ¼ 3 054, 2 932, 2 226, 1 654, 1 476, 1 359, 1 170, 1 058, 954 cm1.1H NMR (DMSO-d6, 300 MHz): d ¼ 3.81 (s, 12H), 5.23 (s, 4H), 6.71 (s, 2H), 7.14 (s, 4H), 7.26 (d, 2H), 7.67 (d, 2H), 7.62–8.37 (m, 18H, pyrene H). MALDI-TOF MS m/z ¼ 929 [Mþ_{]. C} 62H44N2O7: Calcd. C 80.16, H 4.77, N 3.02; Found C 80.02, H 4.68, N 2.90.

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General Procedure for the Synthesis of Dendrons (3),

(5) and (7)

A 250 mL three-necked round-bottomed flask equipped with a magnetic stir bar, reflux condenser, thermometer and addition funnel containing dry THF was flushed with nitrogen and charged with 50 mL of dry THF, and LiAlH4(2.0 equiv.) whereupon a vigorous

gas evolution was observed. Then, a solution of appropriate pyrene-substituted methyl benzoate (1.0 equiv.) in 150 mL of THF was added drop wise for 1.0 h at 0 8C. After the addition was over, the flask was heated to reflux temperature overnight. Excess reducing agent was cautiously destroyed by drop wise addition of 15 mL of dilute HCl at 0 8C. Then, the solvents were removed under vacuum, and the residue was extracted three times with 300 mL of CH2Cl2.

After drying with Na2SO4, the extract was evaporated to dryness.

The crude product was purified as outlined in the following text.

Synthesis of Pyrene G1-CH

2

OH (3)

This was prepared by the reduction of pyrene-G1-COOMe (2) using LiAlH4, purified by column chromatography eluting with CH2Cl2/

hexane/THF ¼ 60:40:1.Yield 89%.

IR (KBr): n ¼ 3 568, 3 086, 3 059, 3 043, 2 928, 2 878, 1 587, 1 456, 1 367, 1 167, 1 054 cm1_.1_{H NMR (DMSO-d}

6, 300 MHz): d ¼ 4.30 (d,

2H), 4.99 (b, OH), 5.23 (s, 4H), 6.96 (t, 1H), 7.18 (d, 2H), 8.02–8.38 (m, 18H, pyrene H). MS (70 eV, EI): m/z ¼ 568 [Mþ]. C41H28O3: Calcd.

C 86.60, H 4.96, O 8.44; Found C 86.43, H 4.75.

Synthesis of Pyrene G2-CH

2

OH (5)

This was prepared by reducing pyrene-G2-COOMe (4) using LiAlH4,

purified by column chromatography eluting with CH2Cl2/hexane/

THF ¼ 65:20:5. Yield 78%. IR (KBr): n ¼ 3 569, 3 078, 3 067, 3 038, 2 934, 2 876, 1 578, 1 465, 1 370, 1 159, 1 049 cm1_.1_{H NMR (DMSO-d} 6, 300 MHz): d ¼ 4.31 (s, 2H), 5.51 (s, 8H), 5.56 (s, 4H), 6.08 (s, 3H), 6.18 (s, 6H), 8.02–8.38 (m, 36H, pyrene H). MALDI-TOF MS m/z ¼ 1 240 [Mþ_{]. C} 89H60O7: Calcd. C 86.11, H 4.87, O 9.02; Found C 85.81, H 4.74.

Synthesis of Pyrene G3-CH

2

OH (7)

This was prepared by reducing the pyrene-G3-COOMe (6) using LiAlH4, purified by column chromatography eluting with CH2Cl2/

hexane/THF ¼ 50:10:10 to give pyrene G3-CH2OH (7) Yield 70%.

IR (KBr): n ¼ 3 578, 3 076, 3 067, 3 028, 2 930, 2 870, 1 576, 1 467, 1 372, 1 165, 1 054 cm1.1H NMR (DMSO-d6, 300 MHz): d ¼ 4.33 (s, 2H),

5.02 (s, 2H), 5.23 (d, 16H), 5.48 (s, 8H), 6.07–6.20 (m, 21H), 8.02–8.38 (m, 72H, pyrene H). MALDI-TOF MS m/z ¼ 2 584 [Mþ]. C185H124O15:

Calcd. C 85.89, H 4.83, O 9.28; Found C 85.67, H 4.76.

Results and Discussion

Synthesis of Dendrons, Core and Dendrimers

The dendrimers reported in this paper consist of pyrene

moieties as the energy donor at the periphery,

3,5-dihydroxybenzyl alcohol units as the non-conjugated

repeat unit and DCM-based chromophore units as the core.

Synthesis of these dendrimers was approached in a

modular fashion using the convergent approach. Thus,

the pyrene containing dendrons and the DCM-based core

chromophore containing two phenolic functionalities were

synthesized separately and then assembled in the last step.

Scheme 1 describes the synthesis of G1 (3), G2 (5) and

G3-dendrons (7), which are building components for the

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construction of G1 (10), G2 (11) and G3 dendrimers (12),

respectively. Synthesis of peripheral unit (1) was achieved

by the reduction of pyrene 1-carboxaldehyde using NaBH

4

.

This compound was then used in combination with

3,5-dihydroxymethyl benzoate for the further elaboration in to

the dendrons. Since Mitsunobu etherification reaction and

other functional group transformations involved in the

synthesis of G2 and G3 dendrons were similar to that

involved in the synthesis of G1 dendron, the experimental

procedures adopted for the latter case were replicated for

the former cases.

Scheme 2 describes the synthesis of core molecule and

bidendron dendrimers up to third generation.

2-(2,6-bis{2-

[(4-hydroxy-3,5-dimethoxy)phenyl]vinyl}-4H-pyran-4-yli-dene)malononitrile (9) was chosen as a symmetrical

bifunctional core molecule, because it can form benzyl

ether spacer group when condensing with dendrons.

Synthesis of core (9) was achieved by two step procedures.

In the first step, pyrone and malononitrile were condensed

in acetic anhydride under reflux condition and this reaction

afforded

(2,6-dimethyl-4H-pyran-4-ylidine)propanedini-trile (8) in 68% yield. The second step involved Knoevenagel

condensation of 3,5-dimethoxy-4-hydroxybenzaldehyde

with

(2,6-dimethyl-4H-pyran-4-ylidine)propanedinitrile

(8), using piperidine as catalyst and CH

3

CN as solvent

resulted the phenolic chromophore core (9) in 86% yield.

Finally the benzyl alcohol functionalized G1 (3), G2 (5) and

G3 dendrons (7) were treated with core (9) applying

identical experimental conditions to obtain G1 (10), G2

(11) and G3 (12) dendrimers, respectively. The time required

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for the completion of the reaction was increased with

increasing the generation number of dendron used. For

instance, the formation of model compound (13) (Scheme 3)

and G1 dendrimer needed 24 h, while G2 and G3 dendrimers

were obtained over 32 and 48 h respectively. In the case of

G3-dendrimer, even if the reaction time was prolonged to

48 h, a small quantity of core chromophore (9) and G3

dendron (7) precursors were still remained. All the

dendrimers were purified by simple column

chromato-graphic technique.

Characterization

The structures of the G1, G2, G3-pyrene-CH

2

OH dendrons (3,

5, 7), their precursors G1, G2, G3-pyrene-COOMe (2, 4, 6), G1,

G2, G3 dendrimers (10, 11, 12), and model compound (13),

were confirmed by FTIR,

1

H NMR, MALDI-TOF MS spectra

and elemental analyses. Figure 1 shows the representative

FTIR spectrum of benzyl ether dendrimer with pyrene end

groups. The absorption peak at around 2 918 cm

1

corre-sponds to CH stretching vibration of saturated

hydro-carbon, the absorption peak at around 2 208 cm

1

corre-sponds to C N stretching vibration and weak absorption

peaks at around 3 061 cm

1

and relatively strong

absorp-tion at around 943 cm

1

corresponds to CH stretching and

out of plane bending motions of trans-vinylene,

respec-tively.

[24,25]

_The

1

_{H NMR spectra show well separated and}

clearly assignable signals for all types of protons including

aromatic pyrene protons as well as benzylic and methoxy

protons in the dendrons and core moieties, respectively. All

the dendrons and dendrimers show characteristic benzylic

protons between d ¼ 4.34 and 5.57. G1 dendron (3), G2

dendron (5) and G3 dendron (7) exhibited a peak around

d

¼ 5.41 which is characteristic of the benzylic proton of the

benzyl alcohol group present at the focal point. The

resonances of the DCM protons, vinylic protons, methoxy

protons present in the core molecule of G1–G3 dendrimers

were observed distinctly as a singlet at d ¼ 6.75, two

doublets at d ¼ 7.29 and 7.69 and a singlet at d ¼ 3.83,

respectively (Figure 2a–c), they were absent in the

dendrons. For higher generation dendrimers, all aromatic

signals could not be distinguished due to strong signal

overlap, but the intensity ratios between aromatic and

aliphatic signals were well matched to the expected value.

The monodispersity of the described dendrimers could

easily be verified by MALDI-TOF mass spectrometry. The

calculated and experimentally determined m/z values were

in good agreement for all the dendrimers, even for third

generation dendrimer with a molecular weight of

5 638 g mol

1

. All the dendrimers exhibited the expected

molecular ion peak as [M

þ

] or [M

þ

þ H

þ

] or [M

þ

þ 2H

þ

]

(Figure 3a–c) and peaks arising from incomplete

substitu-tion reacsubstitu-tions were not detected. The purity of dendrimers

was also confirmed by GPC using THF as eluent. As shown in

Figure 4, the retention time decreases gradually with the

increasing molecular weight from G1 to G3 dendrimer, and

all the peaks were symmetrical and monomodal with a

polydispersity index of 1.1–1.2.

The thermal stability of the dendrimers was studied by

TGA and the thermograms are given in Figure 5. It was

observed that on increasing the generation from G1 to G3,

the thermal stability was also found to increase. The G2 and

G3 dendrimers exhibit good thermal stability with an onset

of degradation temperature at around 400 8C. The amount

of char yield was found to be increased from G1 to G3

dendrimer and this observation indicates that there will not

be any bond cleavage in dendron portion, but CO bond of

core molecule may be cleaved at degradation temperature

Scheme 3. Synthesis of model compound.

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range. Good thermal stability is an important requirement

for the application of the dendrimers in flat panel displays.

Notably, the dendrimers synthesized with pyrene bear

no alkyl groups, but they were soluble in common organic

solvents such as THF, CH

3

CN, dimethylformamide (DMF),

dimethylacetamide (DMAc), N-methylpyrrolidone (NMP)

and dimethyl sulfoxide (DMSO). This can be attributed to

the presence of methoxy groups and ether functionality in

the core and the backbone of the dendrons, respectively.

Photophysical Properties

We used both absorption and emission spectroscopy for the

photophysical characterization of the dendrons,

dendri-mers, model compound and core, and the results like

l

max(abs)

, l

max(flu)

, molar extinction coefficient (e) and

fluorescence quantum yield (F

flu

) measured at two

different temperatures are summarized in Table 1.

Absorp-Figure 2.1_{H NMR spectrum of (a) G1, (b) G2, and (c) G3 dendrimer}

in DMSO-d6.

Figure 3. MALDI-TOF MS spectrum of (a) G1-dendrimer (b) G2-dendrimer and (c) G3-G2-dendrimer.

(8)

tion spectra of compounds 1 (pyrene), 9 (DCM core) and 13

(model compound) are shown in fFigure 6. The spectrum of

model compound 13 shows three bands at 314, 328, 344 nm

and a broad band at 449 nm due to the presence of pyrene

and DCM moieties respectively in the structure (Table 1);

this spectrum is very well approximated by a weighted sum

of compounds 1 and 9. Similar pattern was also observed for

G1–G3 dendrimers (Figure 7). This provides an evidence for

the lack of direct electronic communications between the

pyrene periphery and DCM core in the ground electronic

state.

When comparing the absorption spectra of 1 (pyrene)

and G1 through G3 dendron, it was found that the

absorbance of these compounds increased steadily with

increasing generation number (figure not given). This is

attributed to the increasing number of pyrene units with

generation. The absorption spectrum of G3 dendron (7)

[e (343 nm) ¼ 140 000 L mol

1

cm

1

] was virtually

iden-tical in both shape and intensity to that of compound 1

(pyrene) [e (343 nm) ¼ 30 000 L mol

1

cm

1

_{]; the}

multi-plying factor for the compound 1 was 4.66. This observation

indicates that there is no significant difference in ground

state interactions among the pyrenyl chromophores in

compound 1/G1/G2/G3 dendrons.

Emission spectra of compounds 1 (pyrene) and G1/G2/G3

dendrons are given in Figure 8 along with absorption

spectrum of compound 9 (core); all these spectra were

recorded at 10

6 M

concentration. The emission spectra

Figure 4. GPC traces of (1) G3, (2) G2, and (3) G1 dendrimer. Figure 5. TGA thermograms of G1, G2 and G3 dendrimers.

Table 1. Optical properties of dendrons, dendrimers and model compounds.

Compound

l

max (abs)a)

l

max (flu)

e

b)

F

fluc)

nm

L mol

1

cm

1

25 -C

50 -C

G1

dendron

312, 326, 344

377, 396

d)

74 200

15.5

e)

–

G2

dendron

312, 329, 343

377, 396

d)

118 000

18.8

e)

–

G3

dendron

312, 328, 344

377, 396

d)

140 000

19.0

e)

–

core

448

570

f)

80 000

21.3

g)

–

G1

dendrimer

313, 328, 343, 449

570

f)

199 000

14.0

g)

18.6 G2

dendrimer

312, 328, 344, 448

570

f)

224 000

15.6

g)

19.5 G3

dendrimer

313, 327, 345, 448

571

f)

_{245 000}

_13.0

_18.3

model compound

314, 328, 344, 449

570

f)

_{156 900}

_27.0

g)

_–

a)

All spectra were recorded in THF at a concentration of 106M;b)In THF;c)The quantum yields of the compounds in THF were determined using a solution of quinine sulfate (105Min 0.1MH2SO4solution, having a quantum yield of 0.55) as a standard;d)Spectra were recorded

in THF at a concentration of 106

Mby excitation with 344 nm light;e)lex¼ 344 nm;f)Spectra were recorded in THF at a concentration

(9)

given in this figure does not show any characteristic peak

for excimer formation in dendrons.

[26]

The overlap between

the emission and absorption spectra indicates that Forster

energy transfer is possible from pyrene to the DCM core. To

study the energy transfer in compounds 13 (model

compound), 10, 11 and 12 (G1–G3 dendrimers), emission

spectra of these compounds were recorded [excitation

wavelength used was 444 nm which corresponds to l

max (abs)

of the core] and compared with the respective

absorption spectra. It was observed that the emission

intensity at 573 nm increased from model compound (13)

through G3 dendrimer (Figure 9). Increase in fluorescence

intensity from G1 to G2 dendrimer is high compared to that

of G2 to G3 dendrimer. The low increase in the fluorescence

intensity of later molecule is due to the large distance

between core and end group of G3 dendrimer.

[27]

Emission

from pyrene at about 377 or 396 nm was not found which

implied that the emission of pyrene was almost quenched

in these dendrimers. This can be attributed to the fact that

there is an efficient energy transfer from the pyrene

molecules present at the periphery of the dendrimers which

acts as donor components to the DCM molecule present in

the core which acts as the acceptor components. It is worthy

to mention here that the emission spectra obtained for

model compound and dendrimers when exciting these

molecules at 344 nm [l

max (abs)

of pyrene] and 444 nm (for

core) were virtually identical to that of given in Figure 9.

Figure 6. Absorption spectra of compounds 1, 9 and 13. All the spectra were recorded in THF at a concentration of 106

M.

Figure 7. Absorption spectra of G1, G2 and G3 dendrimers. All the spectra were recorded in THF at a concentration of 106M.

Figure 9. Emission spectra of dendrimers and model compound 13. All spectra were recorded in THF at a concentration of 106

M by excitation with 444 nm light.

Figure 8. Emission and absorption spectra of compounds 1, G1– G3 dendrons and 9, respectively. Note the overlap between the emission spectra of 1, G1–G3 dendrons with the absorption spectrum of 9. All the spectra were recorded in THF at a concen-tration of 106

(10)

The fluorescence quantum yield (F

flu

) of all the

compounds were found to be ranged from 13 to 27% in

THF. No significant correlation between quantum yield and

generation number can be deduced from these values. The

low quantum yield of dendrimers compared to that of

model compound may be due to more efficient self

quenching by non-radiative pathway.

Fluorescence spectroscopy is a well-established method

for detecting aggregated molecules at various

tempera-tures. It has been reported that the absorption and emission

spectrum will be changed when micellization occurs.

[28]

_In

order to study the effect of temperature on aggregate

formation in pyrene based dendrimers, we measured the

absorption and emission spectra of these dendrimers at two

different temperatures viz. 25 and 50 8C at a concentration

of 10

6M

in trichloroethane (Figure 10). Careful analysis and

comparison of these spectra with a relevant reported

work

[26]

gave a conclusion that there was no aggregation in

the dendrimers at the concentration studied. The quantum

yield measured at 50 8C for G1–G3 dendrimers are also

included in the Table 1 and found that the values are high as

expected due to more number of molecules undergo

excitation upon heating.

Conclusion

In the present study, a novel class of dendrimers containing

a 2-pyran-4-ylidene-malononitrile (DCM) moiety as the

centre chromophore and pyrene as the peripheral

chromo-phore were designed and synthesized through the

Mitsu-nobu coupling reaction in high yield up to third generation.

These dendrimers shown excellent thermal stability and

good red emission properties. It was found that the energy

transfer takes place from donor chromophore (i.e., pyrene

molecules) present at the periphery to the acceptor

chromophore (i.e., DCM moiety) present at the centre of

the dendrimers. The emission intensity corresponding to

DCM moiety was found to be increased with increasing the

generation number of the dendron attached to it.

Acknowledgements: M. V. thanks the National Science Council (NSC) of Taiwan for financial support.

Received: October 28, 2010; Revised: January 7, 2011; Published online: February 21, 2011; DOI: 10.1002/macp.201000674 Keywords: dendrimers; fluorescence; FRET; light harvesting; star polymers

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