Synthesis, linear and nonlinear optical properties of thermally stable ferrocene-diketopyrrolopyrrole dyads

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ARTICLE

Synthesis, linear and nonlinear optical properties of thermally

stable ferrocene-diketopyrrolopyrrole dyads

Received 00th January 20xx,

Accepted 00th January 20xx

a

b

*,a

b

Sarbjeet Kaur, Sugandha Dhoun, Griet Depotter, Paramjit Kaur, Koen Clays and Kamaljit

*

,a

Singh

DOI: 10.1039/x0xx00000x

A set of new ferrocene-diketopyrrolopyrrole (Fc-DPP) conjugated dyads was synthesized and their optical, nonlinear

optical (NLO) and electrochemical properties were investigated. The second-order nonlinear polarizabilities were

determined using Hyper-Rayleigh Scattering with femtosecond pulsed laser light at 840 nm. The dyads depicted structure

dependent NLO response, which could be explained by correlating optical as well as electrochemical data. In the latter

case, it is shown that the amplitude of the Fc based one-electron redox process of D-π-A type dyads is doubled in the

dyads of the type D-π-A-π-D, where the acceptor (DPP) is flanked by two Fc donors.

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INTRODUCTION

π-bridge length) has more impact (greater red shift of the

absorption band and smaller optical band gap) on the NLO response

23

than by modulating the acceptor strength. As a consequence, the

Fc based (D-π-A) compounds with shortest conjugation path

showed higher intrinsic hyperpolarizability. Additionally, owing to

their reversible redox behaviour, these chromophores recorded

different hyperpolarizability values in each of the two redox states

and a high on/off (βon/βoff) ratio. Among the various known

1

-5

Materials exhibiting nonlinear optical (NLO) response are of great

interest for the development of optical devices for applications in

6

-7

8

9-12

the field of photonics, nanophotonics and optoelectronics

such as optical signal processing, broad band optical

communications, integrated optics, optical sensing, optical poling,

optical limiting, optical computing etc. Standard materials generally

contain a donor-acceptor combination linked by a conjugated

32

33

acceptors such as thiazole,

and benzodiazthiazole,

34-36

1

3-15

diketopyrrolopyrrole unit

for optoelectronics and organic photovoltaics

has emerged a promising candidate

bridge.

The NLO response of ferrocene (Fc) based dyads has

37

34,38

such as organic

been a subject of numerous investigations owing to many attractive

39-41

1

6-18

light emitting diodes (OLEDs),

organic field effect transistors

features of this organometallic species.

Depending upon the

42-44

OFETs),

45-47

(

organic solar cells (OSCs),

dye sensitized solar cells

oxidation state of the metal centre, the Fc unit can turn a strong

donor or acceptor, a feature that has also been exploited in the

reversible redox switching of the NLO response in Fc dyads.

48-49

DSSCs)

etc. This sub-unit has two amide groups that make it a

1

9-24

strong acceptor, and consequently the energy of the lowest

unoccupied molecular orbital (LUMO) of D-A or D-π-A systems,

wherein appropriately substituted diketopyrrolopyrrole is used as

Ferrocene dyads generally possess low oxidation potential and

upon facile charge transfer (CT) to an acceptor yield stable α-

Further, as a specific feature of the

substitution pattern of the Fc unit, the non-centrosymmetric dyads

50

17,25-26

acceptor, is considerably lowered. In addition to a good acceptor,

ferrocenyl carbocations.

51

this planar, conjugated bicyclic core also possesses exceptionally

34

22,27-29

high photochemical, mechanical and thermal stability, thus

are associated with high optical nonlinearities.

Suitably

rendering it a good candidate for π-conjugated donor-acceptor (D-

A) dyads. To the best of our knowledge, the second-order nonlinear

functionalized push-pull dyads (D-π-A) in which electron donor (D)

Fc is connected by π-conjugating spacers to strong electron

acceptors (A), have witnessed significant interest in their synthesis,

owing to their structure dependent electrochemical, optical and

polarizability (β) of diketopyrrolopyrrole-based dyads has not been

explored yet.

28,30-31

nonlinear optical properties.

Correlating the absorption,

electrochemical, theoretical calculations and Hyper-Rayleigh

Scattering (HRS) experiments, we earlier demonstrated that

varying the conjugation pathway between D (Fc) and A (increasing

In this work, new dyads having a Fc donor and an appropriately

2

3

substituted 2,5-bis-(n-decyl)-3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-

50,52-55

1

,4(2H,5H)-dione (DPP) acceptor,

intercepted by a conjugated

linker (Figure 1, A and B), representing D-A or D-A-D type design in

which the acceptor is flanked by two Fc units through a conjugated

bridge (Figure 1, C) have been prepared. Their structure is

1

13

a

determined by means of microanalytical data, H and C NMR, UV-

visible, and FTIR spectroscopy. The NLO properties have been

determined in THF solution by means of the HRS technique (under

femtosecond pulsed 840 nm laser light). The experimental linear

Department of Chemistry, UGC-Centre of Advance Study-II, Guru Nanak Dev

University, Amritsar-143005, India. Email: kamaljit.chem@gndu.ac.in

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reported as follows: chemical shift in ppm (δ), integration,

multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet,

R¹br = broad) and coupling constant J (Hz). The purity of the

compounds was determined by elemental analysis carried out

on Thermoscientific FLASH 2000 organic elemental analyzer

O

N

CH

2

(CH

2

)

8

CH

3

O

N

CH

2

(CH

2 8 3

) CH

N

O

N

O

R¹

O

Fe

H

3

C(H

2

C)

8

H

2

C

H

3

C(H

2

8 2

C) H C

NC

CN

and was within

was recorded on Perkin-Elmer FTIR-C92035 Fourier-transform

±0.4% of the theoretical values. IR spectrum

(

A) R¹= CHO, CH=C(CN)

2

and

(B) R¹= Br

-1

spectrophotometer in range 400–4000 cm as KBr pellets. All

reported yields are isolated yields. Melting points were

recorded in open capillaries and are uncorrected. For column

chromatography, silica gel (60–120 mesh) and/or neutral

alumina were employed and eluted with ethyl acetate/hexane

O

N

CH

2

(CH

2 8 3

) CH

N

O

R¹

O

C)

H

3

C(H

2

8 2

H C

_C₎_R1 = ferrocene (Fc), Fc-C

6 4

H -

or

measurements were made using CHI660D electrochemical

chloroform/hexane

mixtures.

Electrochemical

(

Figure 1. Chemical structures of Fc-DPP dyads: D-A (A, B), D-A-D (C). workstation using three electrodes- platinum as working as

well as counter electrode and Ag/AgCl as reference electrode.

The experiments were carried at 1 x 10 M solution of the

-4

-2

compound in dichloromethane using

tetrabutylammoniumhexafluorophosphate as supporting

2

x

10

M

electrolyte. The solutions were purged with nitrogen for 10

optical properties of the above derivatives have also been min and the working electrode as well as the reference

computed by employing density functional theory (DFT) calculations electrode was cleaned after each reading. The experiments

-1

and revealed a good correlation between the experimental results were carried out at scan rate of 100 mVs . Thermogravimetric

and the theoretically calculated data. Energies of the frontier analysis were recorded on TGA/DSC 1 STAR SYSTEM FROM

molecular orbitals (FMOs) have been computed from time

METTLER TOLEDO in the temperature range of 0-800 °C at the

-

1

dependent DFT calculations and spectral resolution has been

achieved by band fitting. Moreover, the structure-polarization

heating rate of 10

Femtosecond HRS measurements

°C min under nitrogen atmosphere.

59-64

were performed at 840

(

dipole moment) relationship has been analysed and the effect of

nm using a commercial Ti: sapphire laser at ambient

temperature. Crystal Violet in methanol was used as the

reference, with a value of 434 × 10 esu at 840 nm for the

octopolar βxxx hyperpolarizability tensor component.

the molecular design on the observed linear, electrochemical and

NLO properties has been discussed.

–30

EXPERIMENTAL SECTION

Computational details

Materials and reagents

Theoretical calculations were carried out by using the Gaussian

0

6

9 suite of programs. Optimization of molecular geometries

5

All liquid reagents were dried/purified by using the of all the chromophores and related calculations were

recommended drying agents and/or distilled over 4 Å performed by density functional theory (DFT) method using

molecular sieves. Tetrahydrofuran was dried using sodium B3LYP functional group and 6-31G as the basis set. The first 15-

metal/benzophenone, while chloroform and dichloromethane 30 excited states were calculated by using time-dependent

were dried over fused CaCl

were distilled and stored over KOH under nitrogen. N,N- well in dichloromethane as solvent using CPCM model. The

Dimethylformamide was distilled over CaH and stored over 4Å molecular orbital contours were plotted using Gauss view

2-(3,5,5-trimethyl-2- 5.0.9.

2

. Triethylamine and piperidine density functional theory (TD-DFT calculations) in gas phase as

2

5

6

molecular sieves. Acetyl ferrocene 6a

cyclohexenylidene)malononitrile 10

bistriphenylphosphinedichloropalladium (II) were prepared

,

5

7

and

5

8

following the reported procedures. Ferrocene, phosphorous Synthesis of dyads and intermediates

oxychloride, 1-bromodecane, bromine, 4-aminoacetophenone

and malononitrile were purchased from Spectrochem and

used as received. K CO was dried at 120 C overnight in a

2 3

furnace. 2-Furonitrile, 2-methyl-1-butanol and copper (I)

iodide were purchased from SIGMA ALDRICH and used as such.

o

Synthesis of

(2H,5H)-dione 1

3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4-

Nitrogen was purged (15 min) in 2-methyl-1-butanol (60 ml)

contained in a three-neck round bottomed flask (250 ml)

equipped with a reflux condenser. Sodium metal (3.45 g, 150

Instrumentation

mmol) and FeCl

3

(0.05 g, 0.31 mmol) were added and the

C until sodium metal had

Spectrophotometer. H NMR and C NMR spectra were completely reacted (indicated by complete dissolution). The

recorded on Bruker Biospin Avance III HD at 500 MHz, in CDCl

reaction mixture was cooled to 85 C and 2-furonitrile (9.31 g,

and/or DMSO-d containing TMS as internal standard. Data are 100 mmol) was added followed by dropwise addition of

UV-visible studies were carried out using HITACHI U-2910 reaction mixture was heated at 90

°

1

13

°

3

6

2

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diisopropyl succinate (8.10 g, 40 mmol) over a period of 1 h. 1.42 (m, 28H, -CH ), 1.66-1.72 (m, 4H, -CH ), 4.06 (t, J= 7.5 Hz,

2

Reaction mixture was heated at 85

was cooled to 50

°

2

C for 2 h, after which it 4H, -CH ), 6.62 (d, J = 5 Hz, 2H, furanyl C3-CH) and 8.25 (d, J = 5

1

3

ο

°

C and 50 ml methanol was added. The Hz, 2H, furanyl C4-CH). C NMR (125 MHz, CDCl , 25 C): δ

3

reaction mixture was neutralized using glacial acetic acid and (ppm) 14.13, 22.69, 26.86, 29.28, 29.30, 29.53, 29.59, 30.20,

stirred for 15 min. and then cooled to ambient temperature 31.90, 42.50, 106.28, 115.51, 122.10, 126.41, 132.51, 146.17

and the contents were filtered over sintered glass (G4) funnel. and 160.52. Anal. Calcd. for C34

Residue was washed twice with hot methanol and de-ionized 3.96. Found: C, 57.89; H, 6.59; N, 3.95.

water to yield analytically pure dark red solid (61%). Mp>

2 2 4

H46Br N O : C, 57.80; H, 6.56; N,

1

o

-1 1

.

300 C IR (KBr): νmax1628, 1648, 3153 and 3417 cm

MHz, DMSO-d , 25 C): δ (ppm) 5.97 (s, 2H, furanyl C4-CH),

H (500

ο

Synthesis of 5-(2,5-bis-(n-decyl)-4-(furan-2-yl)-3,6-dioxo-

,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)furan-2-

carbaldehyde 4

6

2

6

.79 (s, 2H, furanyl C3-CH), 7.18 (s, 2H, furanyl C2-CH) and

1

3

ο

1

0.33 (br, 2H, -NH). C NMR (125 MHz, CDCl

08.05, 114.15, 117.24, 131.74, 144.22, 147.33 and 161.63.

: C, 62.69; H, 3.01; N, 10.44. Found: C,

3

, 25 C): δ (ppm)

Anal. Cald. for C14

8 2 4

H N O

Vilsmeier-Haack formylation of

2

was performed by mixing

(0.38 g, 2.48 mmol) and

C for 1 h to provide the red coloured

chloroiminium ion (Vilsmeier reagent). Solution of (0.5 g,

.91 mmol) in anhydrous DMF (5 ml) was added to this

62.73; H, 3.00; N, 10.48.

anhydrous DMF (10 ml) and POCl

stirring the solution at 0

3

°

2

Synthesis of 2,5-bis-(n-decyl)-3,6-di(furan-2-yl)pyrrolo-[3,4-

c]pyrrole-1,4-(2H,5H)-dione 2

0

reagent at 0 °C and the reaction warmed and stirred at 100 °C

for 4 h. Subsequent to the completion (TLC), the reaction

(3 g, 11.2 mmol) in anhydrous DMF (250 ml) mixture was cooled and quenched with pre-cooled saturated

under blanket of anhydrous N gas, anhydrous K CO (4.64 g, aqueous solution of sodium acetate and extracted with DCM

3.60 mmol) was added and the reaction mixture was heated (3 x 30 ml). The organic extract was washed with water (2 x 25

To a solution of

1

2

3

at 120

°

C for 1 h. 1-Bromodecane (7.44 g, 33.60 mmol) was ml), dried over anhydrous sodium sulfate and evaporated

under reduced pressure to obtain crude , which was purified

until it completed. DMF was removed under reduced pressure by column chromatography using 5:95 (ethyl acetate/ hexane)

added dropwise and reaction was stirred for 12 h at 120

°

C

4

o

and the residue was extracted with chloroform (3 x 30 ml). The as eluents to obtain dark red solid

4 (50%). Mp 80-82 C IR

-1 1

extract was washed twice with water and the dried (over (KBr): νmax1587, 1668, 2850, 2919 and 3124 cm

.

H (500 MHz,

), 1.25- 1.43

), 4.10- 4.19 (m, 4H, -

), 6.74-6.75 (dd, 1H, furanyl C3’-CH ), 7.41 (d, J = 5 Hz, 1H,

2

(48%). Mp 80-82 C IR furanyl C4’-CH), 7.70 (d, J = 1.5 Hz, 1H, furanyl C2’-CH), 8.34 (d,

ο

anhydrous sodium sulphate) organic layer was evaporated CDCl

under reduced pressure to obtain crude

, which was purified (m, 28H, -CH

by column chromatography using 5:95 (ethyl acetate/hexane) CH

3

, 25 C): δ (ppm) 0.87 (t, J = 7.5 Hz, 6H, -CH

3

2

), 1.68- 1.74 (m, 4H, -CH

2

o

as eluent to obtain dark red solid

KBr): νmax1590, 1669, 2850, 2917, 2955, 3106 and 3155 cm

-

1

(

.

J = 4 Hz, 1H, furanyl C4 -CH), 8.47 (d, J = 5 Hz, 1H, furanyl C3-

1

ο

13

ο

H (500 MHz, CDCl

.25- 1.40 (m, 28H, -CH

m, 4H, -CH

), 6.69-6.70 (dd , 2H, furanyl C3-CH), 7.63 (d, J = 5 31.88, 31.92, 42.77, 106.78, 110.56, 113.90, 119.86, 122.26,

Hz, 2H, furanyl C4-CH) and 8.30 (d, J = 5 Hz, 2H, furanyl C2- 144.30, 146.28, 148.32, 152.77, 160.33, 161.10 and 177.00.

3

, 25 C): δ (ppm) 0.87 (t, J = 5 Hz, 6H,-CH

3

), CH) and 9.74 (s, 1H, -CHO). C NMR (125 MHz, CDCl , 25 C): δ

1

2

), 1.66-1.72 (m, 4H, -CH

2

), 4.04-4.12 (ppm) 14.12, 22.69, 29.30, 29.35, 29.52, 29.58, 29.62, 29.65,

(

2

1

3

ο

CH). C NMR (125 MHz, CDCl

3

48 2 5

, 25 C): δ (ppm) 14.10, 22.68, Anal. Calcd. for C35H N O : C, 69.65; H, 6.68; N, 6.63. Found:

2

5.97, 26.86, 29.12, 29.31, 29.55, 30.22, 31.90, 42.43, 106.48, C, 69.73; H, 6.70; N, 6.66.

13.45, 120.08, 133.67, 144.70, 145.13 and 160.88. Anal.

1

Calcd. for C34

48 2 4

H N O : C, 74.42; H, 8.82; N, 5.10. Found: C,

Synthesis of 5-(4-(5-bromofuran-2-yl)-2,5-bis-(n-decyl)-3,6-

dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)furan-2-

carbaldehyde 5

74.52; H, 8.86; N, 5.09.

Synthesis of 3,6-bis-(5-bromofuran-2-yl)-2,5-bis-(n-decyl)-

pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione 3

A solution of

was cooled to 0

(100 ml) 0.24 mmol) was made and the reaction mixture stirred at 0

(3 g, 5.50 mmol) anhydrous for 2 h. After completion of the reaction (TLC), saturated

3

(150 ml) precooled to 0 C. The reaction mixture was aqueous solution of sodium thiosulfate was introduced to

4

(0.13 g, 0.23 mmol) in anhydrous CHCl

3

(15 ml)

°C and portion-wise addition of NBS (0.043 g,

A solution of bromine (1.93 g, 12.10 mmol) in CHCl

was slowly added to a solution of

3

°C

2

o

CHCl

stirred for completion at the same low temperature and quench the reaction and extracted with chloroform (3 x 25 ml).

treated with saturated aqueous solution of sodium The extract was washed with water (2 x 20 ml), dried over

thiosulphate. The bromine free solution was extracted with anhydrous sodium sulfate and evaporated under reduced

chloroform (3 x 30 ml) and the organic extract was washed pressure to obtain crude

with water (2 x 25 ml). The organic extract was dried over hexane to obtain dark red solid

anhydrous sodium sulphate, filtered and evaporated under (KBr): νmax1584, 1666, 2850, 2920 and 3128 cm

5, which was recrystallized from

o

5

(71%). Mp 140-142 C IR

-1 1

.

H (500 MHz,

), 1.24- 1.44

column chromatography using 30:70 (CHCl /hexane) as eluents (m, 28H, -CH ), 1.69- 1.74 (m, 4H, -CH ), 4.06- 4.18 (m, 4H, -

ο

3 3

reduced pressure to obtain crude 3, which was purified by CDCl , 25 C): δ (ppm) 0.87 (t, J = 7.5 Hz, 6H, -CH

3

2

o

to obtain dark red solid

ν_m_a_x1548, 1584, 1665, 2851, 2919, 2954 and 3128 cm H (500 1H, furanyl C3’ -CH), 8.34 (d, J = 5 Hz, 1H, furanyl C4 -CH), 8.40

2

3 (58%). Mp 138-140 C IR (KBr): CH ), 6.67 (d, J = 5 Hz, 1H, furanyl C4’ -CH), 7.41 (d, J = 5 Hz,

-

1 1

.

ο

, 25 C): δ (ppm) 0.87 (t, J = 7.5 Hz, 6H, -CH

13

C

MHz, CDCl

3

), 1.26- (d, J = 5 Hz, 1H, furanyl C3 -CH) and 9.74 (s, 1H, -CHO).

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ο

NMR (125 MHz, CDCl , 25 C): δ (ppm) 14.12, 22.68, 26.85, 70.84, 72.31, 120.48, 155.29 and 190.82. Anal. Calcd. for

3

2

1

9.26, 29.30, 29.53, 30.16, 30.39, 31.89, 42.63, 42.82, 106.52,

15.92, 120.11, 123.87, 127.76, 134.35, 145.84, 148.15,

52.85, 160.23, 160.87 and 177.03. Anal. Calcd. for

13

C H11ClFeO: C, 56.88; H, 4.04. Found: C, 56.93; H, 4.03.

(4-(2-Formyl-1-chlorovinyl)phenyl)ferrocene 7b. Red solid.

Yield: 97%. Mp 100-102 C IR (KBr): νmax1032, 1591, 1668,

C H BrN O : C, 64.11; H, 7.23; N, 4.27. Found: C, 63.88; H,

2 5

3

5

47

o

6.96; N, 4.43.

-

1

ο

2

860, 2921, 3085 and 3100 cm H (500 MHz, CDCl , 25 C): δ

. 3

(

ppm) 4.05 (s, 5H, Fc), 4.42 (s, 2H, Fc), 4.72 (s, 2H, Fc,), 6.71 (d,

J = 10 Hz, 1H, -CH), 7.53 (d, J = 10 Hz, 2H, -C ), 7.69 (d, J = 5

Hz, 2H, -C ) and 10.23 (d, J = 5 Hz, 1H, -CHO). C NMR (125

MHz, CDCl , 25 C): δ (ppm) 66.87, 69.89, 69.99, 82.97, 123.14,

Synthesis of 4-ferrocenylethynylbenzene 6b

H

6 5

1

3

6 5

H

ο

3

A solution of sodium nitrite (30.36 g, 44.00 mmol) in 20 ml

water precooled to 0 °C was added dropwise to a stirred

solution of 4-aminoacetophenone (3 g, 22.00 mmol) in 2:1

THF/ hydrochloric acid (24 ml) kept at 0 °C and the mixture

126.07, 127.33, 132.40, 144.66 and 191.57. Anal. Calcd. for

C H ClFeO: C, 65.09; H, 4.31. Found: C, 65.04, H, 4.33.

1

9 15

was stirred at 0

diazotization. Separately, ferrocene (6.95 g, 37.40 mmol) was appropriate 7a

added to 48 ml sulphuric acid and the resulting deep blue heated at 110 C for 15 min and 1N NaOH (125 ml) was added

°

C for 30 min to ensure complete General procedure for synthesis of 8a-b. A solution of

/

7b (47.50 mmol) in dioxane (150 ml) was

°

solution of ferrocenium ion was stirred at ambient and reaction mixture was stirred at the same temperature for

temperature for 2 h. The solution of the ferrocenium ion was 1 h. The contents of the reaction were poured into ice,

then poured in crushed ice and warmed to room temperature neutralized with 1N HCl, passed through celite. The filtrate was

after which addition of copper powder (1.82 g) was made. extracted with hexane (3 x 30 ml). The hexane extract was

Addition of the diazonium salt solution prepared as above was dried over anhydrous sodium sulfate and the solvent removed

made dropwise and the reaction mixture stirred for 24 h at under reduced pressure to obtain corresponding crude 8a

room temperature. Ascorbic acid (9.11 g, 51.72 mmol) was respectively, which was purified by column chromatography

/8b,

added to reduce the unreacted ferrocenium ion to ferrocene. using hexane as eluents to isolate pure 8a

Reaction mixture was passed through celite and the filtrate

was extracted with DCM (3 x 30 ml). The DCM extract was then

/8b, respectively.

o

Ethynylferrocene 8a. Orange solid. Yield: 72%. Mp 50-52 C

washed with water (2 x 25 ml), dried over anhydrous sodium

sulfate and the solvent removed under vacuum to obtain

crude 6b, which was purified by column chromatography using

(

Hexane) IR (KBr): νmax1022, 1443, 1639, 2104, 2854, 2924,

089, 3105 and 3280 cm H (500 MHz, CDCl , 25 C): δ (ppm)

. 3

-1

1

ο

3

2

6

.72 (s, 1H, -CH), 4.19 (s, 2H, Fc), 4.21 (s, 5H, Fc) and 4.45 (s,

H, Fc). C NMR (125 MHz, CDCl , 25 C): δ (ppm) 63.87,

10:90 (ethyl acetate/ hexane) as eluents to isolate pure 6b as

orange solid (40%). Mp 156-158 C (Hexane) IR (KBr): ν 1030,

1

3

ο

o

3

max

-1 1

8.74, 70.07, 71.77 and 73.57. Anal. Calcd. for C12

8.62; H, 4.80. Found: C, 68.66; H, 4.82.

H10Fe: C,

1

.

415, 1563, 1602, 1669, 2956, 2997, 3079 and 3104 cm H

ο

(500 MHz, CDCl , 25 C): δ (ppm) 2.60 (s, 3H, -CH ), 4.04 (s, 5H,

Fc), 4.41 (s, 2H, Fc), 4.72 (s, 2H, Fc,), 7.53 (d, J = 10 Hz, 2H, -

3

1

C H ) and 7.88 (d, J = 10 Hz, 2H, -C H ). C NMR (125 MHz,

3

6

5

6 5

ο

(4-Ethynylphenyl)ferrocene 8b. Orange solid. Yield: 65%. Mp

CDCl

3

, 25 C): δ (ppm) 26.49, 66.94, 69.85, 69.89, 125.76,

o

8-70 C (Hexane) IR (KBr): ν 1027, 1435, 1523, 1604, 1667,

6

2

max

-1

128.60, 134.62, 145.52 and 197.54. Anal. Calcd. for C H FeO:

C, 71.08; H, 5.30. Found: C, 71.12; H, 5.32.

1

8

16

1

ο

104, 2924, 3088 and 3286 cm

. 3

H (500 MHz, CDCl , 25 C): δ

(

ppm) 3.10 (s, 1H, -CH), 4.00 (s, 5H, Fc), 4.35 (s, 2H, Fc), 4.65 (s,

1

3

2

1

H, Fc,), 7.44- 7.56 (m, 4H, -C

6

H

5

). C NMR (125 MHz, CDCl

C): δ (ppm) 66.56, 69.39, 69.72, 84.01, 84.09, 125.77,

26.38, 132.15 and 140.47. Anal. Calcd. for C18 14Fe: C, 75.55;

3

,

ο

5

General procedure for synthesis of 7a-b. Vilsmeier-Haack

formylation of 6a and 6b (50 mmol) was performed by mixing

anhydrous DMF (192 mmol) and POCl (192 mmol) and stirring

H

H, 4.93. Found: C, 75.54; H, 4.91.

3

the solution at 0

chloroiminium ion (Vilsmeier reagent). Solution of appropriate

(50 mmol) in anhydrous DMF (30 ml) was added to this

reagent at 0 C and stirred at 0 C for 4 h. Subsequent to the

completion (TLC), the reaction mixture was cooled and

quenched with pre-cooled saturated aqueous solution of

sodium acetate and extracted with ether (3 x 30 ml). The

organic extract was washed with water (2 x 25 ml), dried over

anhydrous sodium sulfate and evaporated under reduced

°C for 1 h to provide the red coloured

General procedure for synthesis of 9a, 9d, 9e and 9f. For the

synthesis of 9a, anhydrous nitrogen gas was filled in a septum

6

°

capped three-neck round bottom flask containing

mmol), CuI (0.006 g, 0.02 mmol)

5

(0.1 g, 0.2

and

bistriphenylphosphinedichloropalladium (II) (0.02 g, 0.02

mmol). A mixture of THF: Et N (1:1, v/v) (10 ml) was added

3

using hypodermic syringe and the reaction was cooled to 0

followed by evacuation and refilling with N gas. A solution of

(0.105 g, 0.80 mmol) in anhydrous THF (5 ml) maintained

under inert atmosphere was added dropwise to the reaction

mixture at 0 C using cannula and the reaction stirred at

°C,

2

pressure to obtain analytically pure solid 7.

8

a

°

(2-Formyl-1-chlorovinyl)ferrocene 7a. Red solid, Yield: 90%.

Mp 72-74 C IR (KBr): νmax1029, 1417, 1617, 2849, 2924 and

o

ambient temperature. After completion (TLC), the reaction

mixture was passed through celite and the bed washed with

DCM (20 ml). The combined filtrate was washed with water (2

x 20 ml) and the organic layer dried over anhydrous sodium

sulfate and the solvent removed under reduced pressure to

-

1

ο

2

4

1

956 cm

. 3

H (500 MHz, CDCl , 25 C): δ (ppm) 4.25 (s, 5H, Fc),

.57 (s, 2H, Fc), 4.75 (s, 2H, Fc), 6.41 (s, 1H, -CH) and 10.10 (s,

H, CHO). C NMR (125 MHz, CDCl , 25 C): δ (ppm) 68.91,

3

1

3

ο

4

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ARTICLE

obtain crude 9a, which was purified by column 2,5-bis-(n-decyl)-3,6-bis-(5-(ferrocenylethynyl)furan-2-

chromatography using 5:95 (ethyl acetate/hexane) as eluents yl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione 9e. Dark blue solid.

o

to isolate analytically pure 9a

.

Yield: 40%. Mp 146-148 C (30:70 (CHCl /hexane)IR (KBr):

3

ν

-1 1

.

max1026, 1583, 1656, 2195, 2852, 2921 and 2953 cm H (500

ο

MHz, CDCl , 25 C): δ (ppm) 0.86 (t, J = 5 Hz, 6H, -CH ), 1.23-

3

Following the above procedure and using

mmol), CuI (0.001 g, 0.008

3

(0.01 g, 0. 14

mmol) and

bistriphenylphosphinedichloropalladium (II) (0.0089 g, 0.01

1

4

6

.45 (m, 28H, -CH

2

), 1.72- 1.78 (m, 4H, -CH

2

), 4.15 (t, J= 7.5 Hz,

H, -CH ), 4.27 (s, 10H, Fc), 4.33 (s, 4H, Fc), 4.56 (s, 4H, Fc),

2

.81 (d, J = 5 Hz, 2H, furanyl C4 -CH) and 8.35 (d, J = 5 Hz, 2H,

1

3

ο

mmol) and 8a (0.0949 g, 0.42 mmol), 9e was obtained.

Similarly, using 3 (0.1 g, 0. 14 mmol), CuI (0.001 g, 0.008 mmol)

and bistriphenylphosphinedichloropalladium (II) (0.008 g, 0.01

mmol) and 8b (0.06 g, 0.21 mmol), 9d and 9f were isolated in

furanyl C3 -CH). C NMR (125 MHz, CDCl , 25 C): δ (ppm)

3

1

4

1

4.13, 22.69, 26.97, 29.36, 29.57, 29.68, 29.70, 30.29, 31.89,

2.59, 62.92, 69.67, 70.25, 71.77, 97.04, 107.23, 117.71,

21.55, 132.35, 140.13, 144.21 and 160.74. Anal. Calcd. for

5

1 % and 17 % yields, respectively. However, when the molar

ratio of the reactants was changed to (0.1 g, 0. 14 mmol), CuI

0.001 g, 0.008 mmol) and

bistriphenylphosphinedichloropalladium (II) (0.008 g, 0.01

mmol) and 8b (0.12 g, 0.42 mmol), 9d and 9f were isolated in 3,6-bis(5-((4-ferrocenyl)phenylethynyl)furan-2-yl)-2,5-bis-(n-

0 % and 40 % yields, respectively. Further, in these reactions decyl)-pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione 9f. Dark blue

C H Fe N O : C, 72.20; H, 6.69; N, 2.90. Found: C, 72.26; H,

5

8

64

2 2 4

3

6.66; N, 2.89.

(

2

o

varying but significant amounts of the corresponding solid. Yield: 40%. Mp 140-142 C (40:60, CHCl /hexane) IR

3

diacetylene product as a consequence of Glaser coupling (KBr): νmax1031, 1553, 1579, 2192, 2852, 2920 and 3094 cm

reaction were isolated.

-1

.

1

ο

H (500 MHz, CDCl

CH ), 1.25- 1.47 (m, 28H, -CH

s, 10H, Fc), 4.17 (t, J = 7.5 Hz, 4H), 4.39 (s, 4H, Fc), 4.68 (s, 4H,

Fc), 6.89 (d, J = 5 Hz, 2H, furanyl C4 -CH), 7.45- 7.49 (m, 8H, -

3

, 25 C): δ (ppm) 0.87 (t, J = 7.5 Hz, 6H, , -

3

2

), 1.74- 1.80 (m, 4H, -CH ), 4.06

2

(

5-(2,5-bis-(n-decyl)-3,6-dioxo-4-(5-ferrocenylethynyl)furan-2-

yl)-2,3,5,6-tetrahydro pyrrolo[3,4-c]pyrrol-1-yl)furan-2-

1

3

6 5

C H ), and 8.39 (d, J = 4 Hz, 2H, furanyl C3 -CH). C NMR (125

o

carbaldehyde 9a. Yield: 45%. Mp 78-80 C (5:95, ethyl

acetate/hexane) IR (KBr): νmax1021, 1583, 1663, 2197, 2852,

ο

, 25 C): δ (ppm) 14.11, 22.70, 25.97, 28.98, 29.12,

MHz, CDCl

3

2

6

1

9.17, 29.31, 29.37, 29.56, 29.71, 31.64, 31.91, 31.94, 33.84,

5.54, 66.64, 66.94, 69.80, 69.85, 114.07, 115.93, 123.51,

24.06, 125.76, 125.93, 128.60, 131.66, 139.28 and 156.91.

-

1 1

.

ο

2

0

(

923, 2958 and 3127 cm

.86 (t, J = 5 Hz, 6H, -CH ), 1.25- 1.45 (m, 28H, -CH

m, 4H, -CH ), 4.14- 4.19 (m, 4H, -CH ), 4.28 (s, 5H, Fc), 4.35 (s,

H (500 MHz, CDCl

3

, 25 C): δ (ppm)

3

2

), 1.71- 1.75

2

2 2 4

Anal. Calcd. for C70H72Fe N O : C, 75.27; H, 6.50; N, 2.51.

Found: C, 75.33; H, 6.52; N, 2.52.

2

H, Fc), 4.57 (s, 2H, Fc), 6.84 (d, J = 5 Hz, 1H, furanyl C4 -CH),

.41 (d, J = 5 Hz, 1H, furanyl C3 -CH), 8.35 (d, J = 4 Hz, 1H,

7

furanyl C4’ -CH), 8.50 (d, J = 5 Hz, 1H, furanyl C3’ -CH) and 9.74

1

s, 1H, -CHO). C NMR (125 MHz, CDCl

3

ο

, 25 C): δ (ppm) 14.13, Synthesis

(

3

of

2-((5-(2,5-bis-(n-decyl)-3,6-dioxo-4-(5-

2

6

1

2.68, 26.88, 26.93, 29.30, 29.35, 29.53, 29.67, 31.88, 42.81, (ferrocenylethynyl)furan-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-

2.61, 69.84, 70.30, 71.85, 98.06, 107.22, 110.88, 117.91, c]pyrrol-1-yl)furan-2-yl)methylene)malononitrile 9b and (E)-

19.86, 123.69, 130.75, 134.66, 141.28, 143.69, 148.40, 2-(3-(2-(5-(2,5-bis-(n-decyl)-3,6-dioxo-4-(5-

52.76, 160.23, 161.10 and 177.00. Anal. Calcd. for (ferrocenylethynyl)furan-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-

: C, 71.93; H, 7.19; N; 3.57. Found: C, 71.98; H, c]pyrrol-1-yl)furan-2-yl)vinyl)-5,5-dimethylcyclohex-2-

enylidene)malononitrile 9c.

C

47

H56FeN

2

O

5

7.22; N, 3.56.

3-(5-Bromofuran-2-yl)-6-(5-((4-

ferocenylphenyl)ethynyl)furan-2-yl)-2,5-bis-(n-decyl)-2,5-

A solution of 9a (0.05 g, 0.06 mmol), piperidine (0.006 g, 0.07

mmol) anhydrous THF (10 ml) under inert atmosphere was

dihydropyrrolo[3,4-c]pyrrole-1,4-dione 9d. Yield: 51%. Mp cooled to 0

°C and a solution of malononitrile (0.008 g, 0.13

/hexane) IR (KBr): νmax1026, 1550, mmol) in anhydrous THF (1 ml) was added dropwise and the

o

1

CDCl

(

30-132 C (50:50, CHCl

580, 1668, 2192, 2851, 2922 and 2953 cm

3

-1 1

.

H (500 MHz, reaction stirred at 0

, 25 C): δ (ppm) 0.86 (t, J = 10 Hz, 6H, , -CH ), 1.25- 1.64 workup as described earlier, solvent was removed under

m, 28H, -CH ), 1.69- 1.76 (m, 4H, , -CH ), 4.06 (s, 5H, Fc), 4.07- reduced pressure to obtain crude 9b, which was further

.16 (m, 4H), 4.38 (s, 2H, Fc), 4.68 (s, 2H, Fc), 6.63 (d, J = 3.5 purified by preparative TLC using 30:70 (CHCl /hexane) to

°C until completion (TLC). After extractive

ο

3

2

4

3

Hz, 1H, furanyl C4’ -CH), 6.88 (d, J = 5Hz, 1H, furanyl C3’ -CH), isolate analytically pure bluish- green solid.

.44- 7.48 (m, 4H, -C ), 8.26 (d, J = 5 Hz, 1H, furanyl C4 -CH)

and 8.36 (d, J = 5 Hz, 1H, furanyl C3 -CH). C NMR (125 MHz,

7

6 5

H

1

3

ο

CDCl

3

, 25 C): δ (ppm) 14.13, 15.00, 22.70, 26.88, 29.28, 29.37, Similarly, using 9a (0.06 g, 0.07 mmol), piperidine (0.008 g,

9.54, 29.69, 31.89, 31.91, 42.51, 48.38, 66.62, 69.65, 69.78, 0.09 mmol) and 2-(3,5,5- trimethyl-2-

3.71, 97.43, 106.70, 107.04, 115.51, 118.31, 118.45, 121.62, cyclohexenylidene)malononitrile 10 (0.03 g, 0.15 mmol) and

22.01, 125.91, 126.31, 131.64, 132.16, 132.76, 139.87, stirring the reaction at 40 C for 24 h furnished 9c as bluish

41.47, 144.56, 146.24, 160.53 and 160.68. Anal. Calcd. for green solid, after extractive work up of the reaction followed

59BrFeN

: C, 68.50; H, 6.52; N, 3.07. Found: C, 68.53; H, by purification as described above.

.54; N, 3.06.

2

8

1

C

6

°

52

H

2 4

O

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ARTICLE

Journal Name

2

-((5-(2,5-bis-(n-decyl)-3,6-dioxo-4-(5-

27.82, 28.00, 29.37, 29.43, 29.52, 29.63, 29.68, 29.71, 30.26,

31.63, 31.93, 31.97, 32.37, 39.09, 42.64, 45.70, 62.88, 69.76,

c]pyrrol-1-yl)furan-2-yl)methylene)malononitrile 9b. Yield: 70.28, 71.81, 78.29, 109.04, 112.39, 113.17, 117.83, 120.60,

(ferrocenylethynyl)furan-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-

o

65%. Mp 100-102 C IR (KBr): νmax1099, 1398, 1531, 1600, 122.32, 124.66, 129.58, 143.99, 145.97, 154.49, 159.70, 160.83

-1 1

1660, 2198, 2228, 2854 and 2925 cm H NMR (500 MHz, and 170.35. Anal. Calcd. for C H FeN O : C, 74.35; H, 7.19; N,

CDCl

.

59 68

4 4

ο

, 25 C): δ (ppm) 0.85- 0.88 (m, 6H, -CH

3

), 1.23- 1.29 (m, 5.88. Found: C, 74.39; H, 7.22; N, 5.86.

28H, -CH ), 1.74- 1.77 (m, 4H, -CH ), 4.16 (t, J = 7.5 Hz, 2H, -

2 2

CH ), 4.25- 4.28 (br, 7H, Fc + 2 x CH ), 4.36 (s, 2H, Fc), 4.57 (s,

2

H, Fc), 6.72 (s, 1H, -CH=), 6.86 (d, J = 5 Hz, 1H, furanyl C4’ - RESULTS AND DISCUSSION

CH), 7.41 (d, J = 10 Hz, 1H, furanyl C3’ -CH), 8.52 (d, J = 5 Hz,

1H, furanyl C4 -CH) and 8.58 (d, J = 5 Hz, 1H, furanyl C3 -CH).

1

3

ο

C NMR (125 MHz, CDCl

3

, 25 C): δ (ppm) 14.11, 22.67, 26.62, Synthesis and characterization

2

4

1

6.92, 29.29, 29.35, 29.46, 29.51, 29.55, 29.66, 29.70, 31.88,

2.22, 42.82, 62.47, 69.95, 70.33, 71.90, 98.76, 107.83,

18.09, 121.63, 124.80, 129.12, 135.27, 141.86, 143.51,

49.07, 150.18, 160.12 and 161.04. Anal. Calcd. for

3

,6-Di(furan-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione

1

61%) was prepared from diisopropyl succinate and 2-

(

furonitrile using a reported method. Dialkylation of

50

1 was

achieved by using n-decylbromide in anhydrous DMF at 120 C

50 4 4

C H56FeN O : C, 72.11; H, 6.78; N, 6.73. Found: C, 72.15; H,

o

6.73; N, 6.75.

5

0

(

Scheme 1). The resultant 2,5-bis-(n-decyl)-3,6-di(furan-2-

yl)pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione (48%) was then

brominated (vide experimental) to obtain 3,6-bis-(5-

bromofuran-2-yl)-2,5-bis-(n-decyl)- pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)dione (58%) using bromine in chloroform.

enylidene)malononitrile 9c. Yield: 60%. Mp 88-90 C IR (KBr): Intermediate was then mono formylated using Vilsmeier-

Haack reaction conditions to obtain 5-(2,5-bis-(n-decyl)-4-

), (furan-2-yl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-

2

(

E)-2-(3-(2-(5-(2,5-bis-(n-decyl)-3,6-dioxo-4-(5-

ferrocenylethynyl)furan-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-

c]pyrrol-1-yl)furan-2-yl)vinyl)-5,5-dimethylcyclohex-2-

3

o

2

-1

ν

max1099, 1384, 1523, 1560, 1662, 2216, 2853 and 2924 cm

.

1

ο

H (500 MHz, CDCl

3

, 25 C): δ (ppm) 0.85-0.88 (m, 6H, , -CH

), 1.23- 1.34 (m, 28H, -CH

), 1.74- 1.78 (m, yl)furan-2-carbaldehyde

), 2.43 (s, 2H, -CH ), 2.61 (s, 2H, -CH

), 4.14- 4.20 (m, bromination of using N-bromosuccinimide (NBS) in

), 4.28 (s, 5H, Fc), 4.34 (s, 2H, Fc), 4.56 (s, 2H, Fc), 6.62 chloroform yielded 5-(4-(5-bromofuran-2-yl)-2,5-bis-(n-decyl)-

2

6

6-67

1.09 (s, 6H, -C(CH

4H, -CH

3

)

2

4

(50%) (Scheme 1).

Subsequent

4

2

(

d, J = 1.5 Hz, 2H, furanyl C4’ -CH), 6.82- 6.84 (m, 2H, -CH=CH-), 3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)furan-2-

68

6

.87 (s, 1H, furanyl C3’ -CH), 8.37 (d, J = 5 Hz, 1H, furanyl C4 - carbaldehyde

5

in good yield (71%) (Scheme 1).

CH) and 8.41 (d, J = 5 Hz, 2H, furanyl C3 -CH). C NMR (125 Ethynylferrocene 8a (72%) and 4-(ethynylphenyl)ferrocene

1

3

69

ο

70

MHz, CDCl

3

, 25 C): δ (ppm) 14.13, 22.69, 25.30, 26.96, 27.16, 8b (65%) were prepared (Scheme 2) using known methods.

Scheme 1. Synthetic route to precursors 2, 3 and 5.

Scheme 2. Synthetic routes to precursors 8a-b.

6

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ARTICLE

CH (CH ) CH

O

N

2

2 8

3

O

N

CH

2

(CH

2

)

8

CH

3

N

O

N

CHO

O

CHO

Fe

O

Pd(II)/ CuI/THF

, rt

Fe

Br

O

+

O

N

2

3 2 8 2

H C(H C) H C

O

3

2

8

2

5

8a

9a

Scheme 3. Synthetic route to 9a.

of 9a with malononitrile or 2-(3,5,5-trimethylcyclohex-2-

Dyads of design A (Figure 1) were easily prepared (Schemes 3-5) by

57

enylidene)malononitrile 10

benzophenoneketyl) and piperidine as base furnished 2-((5-(2,5-bis-

n-decyl)-3,6-dioxo-4-(5-(ferrocenylethynyl)furan-2-yl)-2,3,5,6-

in anhydrous THF (dried over

7

1-72

Sonogashira coupling reaction

of 5-(4-(5-bromofuran-2-yl)-2,5-

bis-(n-decyl-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-

yl)furan-2-carbaldehyde 5 and ethynylferrocene 8a using bis-

triphenylphosphinedichloropalladium (II) and CuI as catalysts

resulting in the formation of 5-(2,5-bis-(n-decyl)-3,6-dioxo-4-(5-

ferrocenylethynyl)furan-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-

(

tetrahydropyrrolo[3,4-c]pyrrol-1-yl)furan-2-

yl)methylene)malononitrile 9b (65%) and (E)-2-(3-(2-(5-(2,5-bis-(n-

decyl)-3,6-dioxo-4-(5-(ferrocenylethynyl)furan-2-yl)-2,3,5,6-

tetrahydropyrrolo[3,4-c]pyrrol-1-yl)furan-2-yl)vinyl)-5,5-

dimethylcyclohex-2-enylidene)malononitrile 9c (60%) (Scheme 4),

respectively. Similarly, Sonogashira coupling of 3 with 8a led to the

c]pyrrol-1-yl)furan-2-carbaldehyde 9a (45%) (Scheme 3). Dyad 9a

served as a common precursor for preparing the other two

2

3

congeners of design A. Thus, Knoevenagal condensation reaction

NC

CN

NC

CN

O

N

CH2(CH2)8CH3

O

N

CH2(CH2)8CH3

O

N

CH2(CN)2

10

O

CN

9a

O

Fe

THF/Piperidine,

o

THF/Piperidine,

0 C, N2

Fe

NC

o

H3C(H2C)8H2C

O

0

C, N2

H3C(H2C)8H2C

O

9

b

9c

Scheme 4. Synthetic route to 9b & 9c.

Scheme 5. Synthetic route to 9e, 9d & 9f.

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ARTICLE

formation of 2,5-bis-(n-decyl)-3,6-bis-(5-(ferrocenylethynyl)furan-2-

yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione 9e (40%) (Scheme 5).

1

.8

.6

.4

.2

0

Finally, Sonogashira coupling of

3 and 8b furnished 3-(5-

bromofuran-2-yl)-6-(5-((4-ferocenylphenyl)ethynyl)furan-2-yl)-2,5-

bis-(n-decyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione 9d (51%)

Ferrocene

5

(Scheme 5) in a synthetically useful manner. Dyad 3,6-bis-(5-((4-

9a

9b

9c

ferrocenyl)phenylethynyl)furan-2-yl)-2,5-bis-(n-decyl)- pyrrolo[3,4-

c]pyrrole-1,4-(2H,5H)-dione 9f (40%) was also isolated (Scheme 5)

along with 9d. All compounds were characterized using spectral

techniques. The thermogravimetric analysis showed that 9a-9c and

250

350

450

550

650

750

850

Wavelength (nm)

9e are thermally stable upto 500 °C whereas, 9d and 9f show

thermal stability upto 200 °C (See SI Figure S1).

Figure 3. Overlay of UV-visible spectra of 9a-9c

dichloromethane at 1 x 10 M and Fc in dichloromethane at 1 x 10

M at 293 K.

& 5 in

-5

-3

UV-visible absorption study

Chromophore 5 and dyads 9a-9f show intense low energy (LE) absorption bands have been resolved using band fitting analysis

charge transfer bands (MLCT or D-A) and weak intensity high energy (See SI Figure S2), although typical LE (MLCT) and HE (ILCT) bands of

(

HE) bands (LMCT or π-π*) in the region of 500-700 nm and 300-450 the pristine Fc merged extensively with the HE and LE bands of the

nm, respectively, which are characteristic bands of DPP based DPP core. However, on the basis of TD-DFT studies (carried using

55,73-79

chromophores.

Owing to extensive overlap, the

B3LYP/ 6-31G basis set and CPCM model using dichloromethane as

65

solvent), apparent contributions of the relevant transitions to the

absorption bands have been assigned (See SI Table S1). The TD-DFT

deduced FMOs of 5 & 9a-9f and the associated energies are

depicted in Figure 2. The position of the LE CT bands of the dyads

show a shift upon varying the strength of the acceptor as well as

upon altering the length of the π-conjugation intervening D and A.

Thus, the dipolar 9a-9c show red shifted LE CT bands (∆λ= 28 nm,

ꢀ

2

ꢀ

2.5

(A)

ꢀ

3

LUMO

3.5

ꢀ3.24714

ꢀ3.22591

ꢀ3.31679

5

→9a, ∆λ= 100 nm, 5→9b, ∆λ= 68 nm, 5→9c) w.r.t. the core 5

owing to the influence of the donor Fc and the acceptor moieties

Figure 3). On increasing the acceptor strength from 9a→9c→9b, a

ꢀ

3.48496

ꢀ

4

∆E= 2.2694

∆E= 2.08248 ∆E= 1.85718

∆E= 1.82344

(

4.5

red shift shift is observed in the LE CT bands (∆λ= 40 nm, 9a→9c,

ꢀ5.14023

HOMO

ꢀ

5

ꢀ

5.30839

ꢀ5.34214

ꢀ

5.51657

5.5

ꢀ

6

1

5

9a

9b

9c

0

.8

.6

0.4

.2

(

B)

Ferrocene

3

ꢀ2

0

ꢀ

2.5

9

d

ꢀ3

LUMO

9e

9f

ꢀ2.94237

ꢀ2.90889

ꢀ2.97829

3.5

0

ꢀ4

∆E= 2.22946

∆E= 2.14236

∆E= 2.08249

4.5

0

ꢀ5.05215

ꢀ5.06078

250

350

450

550

650

750

850

ꢀ5

ꢀ5.17316

HOMO

Wavelength (nm)

5.5

Figure 4. Overlay of UV-visible spectra of 9d-9f

dichloromethane at 1 x 10 M and Fc in dichloromethane at 1 x 10

M at 293 K.

& 3 in

ꢀ

6

-5

-3

9d

9e

9f

Figure 2. Illustration of frontier molecular orbitals of 5, 9a-9c (A);

d-9f (B) at B3LYP/ 6-31G basis set.

9

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∆

λ= 32 nm, 9c→9b). The red shift could be attributed to the is increased by the introduction of a phenyl ring (Figure 4), which

stabilization of the LUMO energy as a consequence of increasing leads to a slight increase in energy of the HOMO in the latter,

the acceptor strength from 9a→9c→9b, whereas the energy of although optical band gap is nearly the same for both of these

HOMO remains nearly the same (Table 1) in all these three dyads. dyads (Table 1). As the extent of π-conjugation is increased from

The experimental linear optical data showed good correlation with 9e→9f, slight red shift is seen in the LE CT band (∆λ= 2 nm, 9e→9f).

the theoretical data. The latter also reveals that on increasing the Theoretical data also provides the evidence for this red shift as the

strength of acceptor, the order of the stabilization of LUMO follows calculated energy of HOMO is nearly the same for both 9e and 9f

the same trend as obtained experimentally (Table 1). The LE CT (Table 1), whereas LUMO of 9f is more stabilized than that of 9e

band of 9d and the disubstituted counterparts 9e and 9f displayed leading to a decreased band gap of the former and the consequent

red shift (∆λ = 28 nm, 3→9d, ∆λ= 52 nm, 3→9e, ∆λ= 54 nm, 3→9f) red shift in the absorption band (Table 1).However, 9d also shows a

w.r.t the position of the absorption band of the core 3, which is red shift as compared to 3 albeit of lower magnitude than observed

attributable to the increase in the extent of π-conjugation. Both 9e in the pair of 9e and 9f. Evidently, the weak electron donor of the Br

and 9f have the same Fc donor, but the intervening π- conjugation group leads to an increased band gap as is also attested from the

Table 1. Comparison of experimental (CV/ UV-visible) and the calculated (TD-DFT) HOMO-LUMO Energy data and dipole moments of 5 &

9

a-9f.

a

Compound

Experimental data

TD-DFT calculations

optical

g

E

(

HOMO

E

LUMO

E

HOMO

E

LUMO

∆E

µ

b

c

d

eV)

(eV)

TD- DFT gas

phase/

(eV)

TD- DFT gas

phase/

(eV)

TD- DFT gas

phase/

TD- DFT gas

phase/

solvent

phase

solvent

phase

solvent

phase

solvent

phase

5

-5.464

-5.012

-5.03

2.066

1.913

1.657

1.675

2.066

1.937

1.943

-3.398

-3.099

-3.373

-3.325

-2.859

-3.075

-2.907

-5.48718/

-3.22808/

-3.24714

-3.10427/

-3.22591

-3.47543/

-3.48496

-3.31788/

-3.31679

-2.75705/

-2.94237

-2.64494/

-2.90889

-2.73256/

-2.97829

2.2591/

2.2694

4.9102/

6.1060

-

5.51657

-5.20853/

5.30839

-5.34948/

5.34214

-5.07601/

5.14023

-4.99301/

5.17316

-4.79628/

5.0515

-4.82267/

5.06078

9

a

b

2.104026/

2.08248

1.87405/

1.85718

1.75813/

1.82344

2.23596/

2.22946

2.15134/

2.14236

2.09011/

2.08249

8.6296/

10.5140

13.2713/

15.7635

11.5754/

13.7962

3.7068/

4.3492

-

9

-

9c

-5.00

-

9

d

e

-4.925

-5.012

-4.85

-

9

0.2513/

0.2931

-

9

f

0.2685/

0.3874

-

a

b

onset

c

optical

g

d

optical

g

B3LYP/6-31G level. Calculated as EHOMO = -e[ Eox

+ 4.4]. Calculated as E

= 1239.84187/λonset

.

Calculated as ELUMO = E

+

E

HOMO

.

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theoretical data (Table 1).

precedence from the literature, wherein the chromophores

possessing MLCT (d→π*) and n→π* transitions, depict an increase

in the dipole moment of the ground state. This leads to

hypsochromic shift of both the transitions on increasing the solvent

polarity and has been ascribed to the electrostatic dipole-dipole

interactions, which stabilize the ground state more than the excited

The dyads 9a-9f show blue shift in LE CT band with the increase in

the polarity of the solvent (See SI Table S2). The concentration

dependence of the extinction coefficients was ruled out as no

significant change in the relative intensity of the absorption bands

was noticed when the absorption spectrum of the dipolar dyads 9b

8

6

-

5

state, resulting in a more dipolar ground state. Also, polar protic

solvents are capable of hydrogen bonding with the available lone

pairs, hence stabilizing the ground state more than the excited

and 9c were recorded at five different concentrations (1.2-6.0 x 10

-5

M (9b) and 0.9-4.7 x 10 M (9c) in THF), which were also used in the

HRS study (vide infra). Thus, the influence of aggregation in

modulating the position of the absorption band and/or intensity has

8

0

state.

been ruled out. Hence, the observed shift in the absorption bands is Electrochemistry

attributed to solvatochromism. However, solvatochromism is only

Electrochemistry was performed in DCM (freshly distilled from

significant in the dipolar dyads 9b and 9c, although it is negative,

-2

CaH

2

), with 2 x 10 M tetrabutyl ammonium hexafluorophosphate

) as a supporting electrolyte (Aldrich, Electrochemical

which suggests existence of a more polar ground state of these

(

TBAPF

6

8

0-82

compounds compared to the corresponding excited states,

grade). A platinum electrode was used both as working as well as

counter electrode and Ag/AgCl as reference electrode (CHI660D

Electrochemical Workstation). All experiments were performed in

suggesting that the ground state is already a strong charge transfer

state represented by the quinoidal forms (Figure 5, 6) as has indeed

8

3

been described for similar dicyanovinyl chromophore. The

2 2

N purged solvent, and a N gas blanket was maintained over the

83-85

marginally higher bond length alternation (BLA)

of 9c (Figure 6)

solution during the experiments. Fc was used as an internal

reference. Voltammograms displayed in the paper were recorded

compared to shorter dyad 9b suggests greater equalization of bond

lengths in the latter, owing to the influence of the stronger

dicyanovinyl acceptor. Thus, the dyads 9b and 9c were expected to

-1

with a scan rate of 100 mVs . Variation in scan rates had minimal

effect on peak potentials as well as inE1/2 values. E1/2 values are

taken as the half-way point between the forward and reverse peak

show higher

β-values in the present series of compounds. The

observed more polar ground states of 9b and 9c also draw

for

each

reversible

redox

process.

2

0

NC

1

9

CN

R

O

7

R

9

17

16

O

18

NC

3

15

1

6

CN

2

3

N

2

3

N

O

13

9

O

1

4

7

1

4

15

1

0

14

O

5

8

1

0

O

5

8

14

Fe

N

6

1

1 12

Fe

O

N

6

1

12

R

O

9

c

9

b

(

where bond lengths (Å): 1→2 = 1.3880; 2→3 = 1.4145; 3→4 =

(where bond lengths (Å): 1→2 = 1.3970; 2→3 = 1.4157; 3→4 =

1.3887; 4→5 = 1.4210; 5→6 = 1.4004; 5→7 = 1.3997; 7→8 = 1.418;

8→9 = 1.3971; 9→10 = 1.4217; 10→11 = 1.3931; 11→12 = 1.4132;

12→13 = 1.3913; 13→14 = 1.4222; 14→15 = 1.3665; 15→16 =

1.4481; 16→17 = 1.3760; 17→18 = 1.4317; 18→19 = 1.3908; 19→20

1

8

1

.3896; 4→5 = 1.4917; 5→6 = 1.3988; 5→7 = 1.4021; 7→8 = 1.4166;

→9 = 1.3966; 9→10 = 1.4230; 10→11 = 1.3944; 11→12 = 1.4104;

2→13 = 1.3936; 13→14= 1.4158; 14→15 = 1.3786; 15→16 =

.4273).

=

1.4274).

Figure 5. DFT/ B3LYP/ 6-31G optimized bond lengths (in Å) of 9b and 9c.

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2 2 8 3

CH (CH ) CH

O

N

CH

2

(CH

2

)

8

CH

C

3

O

N

CH

2

(CH

2

)

8

CH

3

O

N

O

N

O

N

O

b'

O

a

C

a'

O

CN

O

N

_F_e3+

O

C N

Fe

C

NC

b

O

H

3

C(H

2

C)

8

H

2

C

H

3

C(H

2

C)

8

H

2

C

H

3

C(H

2

C)

8

H

(

2

C

N

9

b

(A)

BLA (a

B)

b) = 0.023 Å

BLA (a' b') = 0.035 Å

N

NC

C

c

CN

C N

O

N

CH

2

(CH

2

)

8

CH

3

O

N

2 2 8 3

CH (CH ) CH

N

O

N

C

O

c'

C

Fe

N

H

3

C(H

2

C)

8

H

2

C

O

H

3

C(H

2

C)

8

H

2

C d

O

N

2 2 8 3

CH (CH ) CH

N

O

9c

(C)

d'

O

C

O

BLA (c d) = 0.035 Å

_F_e3+

3 2 8 2

H C(H C) H C

(

D)

BLA (c' d') = 0.037 Å

Figure 6. Depiction of possible quinoid like structures (A-D) of 9b and 9c.

In analogy to the redox behaviour of Fc as well as the core 5, dyads 1.343 V, respectively, in a manner similar to 5, in addition to the

a-9f show electrochemically reversible oxidation peaks (See SI redox wave of Fc donor. Further, the amplitude of the cathodic

9

Figures S10-S16). Dyads 9a and 9b show nearly identical E1/2 values peak of the one Fc unit containing 9a, that correspond to a single

with a negligible cathodic shift (∆E1/2= 0.002V, 9a→9b), although electron redox process was roughly doubled (Table 2) in case of 9e

the acceptor strength in the latter is large. This could be attributed and 9f that contain two Fc units.

to the slight difference in the energies of HOMO of both 9a and 9b

Computational Studies

(Table 1). However, 9c shows a cathodic shift in the oxidation

The molecular geometries of the dyads were optimized on B3LYP/

-31G level along with the TD-DFT calculations using same basis set

potential as compared to 9b (∆E1/2= 0.018V, 9b→9c) (Table 2)

possibly due to decreased acceptor strength, increased intervening

π-conjugation in the former and the marginally higher energy of the

HOMOs (Table 1). Similarly, the disubstituted chromophores also

show cathodic shift in E1/2 on increasing π-conjugation from 9e→9f

6

in gas phase as well as in solvent medium using CPCM model to get

a deeper insight into the effect of varying donor, acceptor and the

extent of π- conjugation on the dipole moment, second-order

6

5

I

II

nonlinear polarizability (β) and other related properties. The

(

∆E1/2 = 0.136 V, ∆E1/2 = 0.099 V) (Table 2) due to rise in energy of

I

calculated HOMO-LUMO band gaps show a good correlation with

the optical band gaps obtained from CV and UV-visible absorption

data (Table 1 & See SI Figure S17). The calculated energies of the

sets HOMO and LUMO (See SI Table S3 & S4) of the dyads show

good correlation with experimental data as discussed in the above

sections. The plots of FMOs (See SI Figure S18) reveal that HOMO-

LUMO band gap is modulated possibly by both the strength of the

acceptors as well as the length of the π-conjugation. Thus, 9b

appended with a stronger dicyanovinyl acceptor shows HOMO-

LUMO band gap (∆E= 1.85718 eV) comparable to 9c (∆E= 1.82344

eV), appended with a relatively weaker acceptor through a longer

π-conjugation bridge even as a minor stabilization of LUMOs of 9f

HOMO of 9f. However, 9d shows anodic shift in E1/2 w.r.t. 9f (∆E1/2

=

II

0

.023 V, ∆E1/2 = 0.034 V 9d→9f) (Table 2), which could be

attributed to the stabilization of HOMO of 9d due to the decreased

π-conjugation in 9d, indicative of an increased electronic

communication between donor and acceptor. Theoretical data also

correlates well with the experimental observation. Further,

whereas in 9a, an additional reversible oxidation wave

corresponding to the core 5 was observed at E1/2 1.148 V, the same

was not observed in both 9b and 9c, which also suggests these

dyads to be efficient D-A systems owing to the increased strength of

the acceptors. Similarly, 9e and 9f represent disubstituted

analogues with D-A-D type constitution and consequently show an

additional reversible oxidation peak at E1/2 1.056V and 0.957 V

(

E

L

= -2.97829 eV) bearing a longer π-conjugation was observed

compared to 9e (E = -2.90889 eV) (Table 1, Figure 2). The contour

L

(Table 2), respectively, attributable to the DPP unit. Dyad 9d

plots of the orbitals involved in the transitions are shown in Figures

however, shows two reversible oxidation peaks at E1/2 0.992 V and

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Journal Name

a

2 2

Table 2. Electrochemical data for Fc, 5 & 9a-9f in CH Cl .

-

6

b

-6

b

Compound

E_p_a(V)

E_p_c(V)

E₁_/₂(V)

i_p_ax 10 (A)

i_p_cx 10 (A)

Fc

5

0.572

1.176

0.472

1.070

1.406

0.640

1.089

0.648

0.635

0.526

0.941

1.293

0.638

1.006

0.516

0.910

0.522

1.123

1.451

0.700

1.148

0.698

0.680

0.581

0.992

1.343

0.694

1.056

0.558

0.957

-2.116

-0.902

-1.897

-0.788

-0.813

-0.074

-0.107

-2.188

-1.596

-2.063

-1.700

-0.759

-0.979

-0.510

-2.180

0.379

-0.279

0.868

-0.295

0.085

0.089

2.091

1.291

-0.938

1.749

0.432

1.892

0.229

1

.496

0.760

.208

9

a

b

1

9

0.748

0.725

0.637

9c

9

d

1

.043

.394

9e

0.751

.106

0.601

.004

1

9

f

1

a

p

Half-wave potential, E1/2 = (Epc + Epa)/2, where Epc and Epa correspond to the cathodic and anodic peak potentials, respectively; ∆E = 80–

-1 b

1

20 mV; and a scan rate of 100 mV s . Amplitudes of the anodic and cathodic peaks.

S19 and S20 (See SI) and tentative assignment to the electronic transitions. The HE transitions are expected to have contributions

transitions is made (See SI Table S1). In the dipolar dyads 9a-9c, LE from multiple transitions i.e. LMCT, π-π*, A→D or intra-ligand CT

transition is assigned as D→A CT transition in which HOMO is transitions (See SI Table S1). Thus, HOMO→LUMO transitions

mainly localised on Fc unit along with small contributions from the correspond to LE CT absorption bands in all the chromophores

π-bridge and DPP unit, whereas the electron density is shifted except in 9a in which H-2→LUMO also contributes to LE CT

towards the acceptor showing the LUMO mainly located on the absorption band. Further, it has been observed that HE transitions

acceptor along with some contributions from the π-bridge and DPP in 9c with relatively large f-values also correspond to D→A CT

unit. However, H-2→LUMO also shows contribution to LE D→A CT transition as visualized from the contour plots of the orbitals (See SI

transition in 9a. Similarly, in the disubstituted chromophores 9e and Figure S19). Thus, the charge transfer is expected to increase due to

9

f, the LE transitions are assigned as HOMO→LUMO (MLCT) the contributions from both HE (H-3→LUMO & H-4→LUMO) and LE

transitions since the HOMO is mainly located on Fc unit with small (HOMO→LUMO) CT transitions, thus

contribution from π-bridge and DPP unit, whereas the LUMO is 9c.

β is expected to be largest for

mainly located on DPP unit with small contribution from π-bridge

The calculated values of dipole moments (µ) in gas phase as well as

in solvent phase correlate well with the structures of the dyads

(the contour plots show increased electron density on the DPP

unit). On the similar basis, LE transition in 9d is assigned as

HOMO→LUMO CT (MLCT) transition. These LE CT transitions show

higher oscillator strength (f) (See SI Table S1) than that of HE

(

Table 1). The central core 5, being a strong acceptor shows a large

dipole moment. On increasing the acceptor strength from

9

a→9c→9b, the dipole moment shows the same trend. Similarly,

1

2 | J. Name., 2012, 00, 1-3

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dipolar analogue 9d also shows dipole moment greater than that of the former has a weaker acceptor as well as smaller dipole moment

9

e and 9f (Table 1). However, the symmetrically substituted 9e and (Table 1). This could be attributed to the high oscillator strength of

f have non-zero dipole moment (Table 1), indicating their non- charge transfer LE absorption band (See SI Table S1) in 9c, as well as

centrosymmetric nature. The optimised geometries of 9e and 9f there is marginal increase in energy of the HOMOs in 9c owing to

Figure S21) show slight distortion from the planarity leading to longer conjugation. Further, in addition to the LE CT absorption

non-zero dipole moments. As 9f is slightly more distorted than 9e, band in 9c, HE band at 306 nm also corresponds to D→A CT

f has slightly larger dipole moment (Table 1). Hence, non-zero transition as depicted from the FMO diagram (See SI Figure S19).

second-order nonlinear polarizability values are expected for both Thus, both HE and LE absorption bands are expected to contribute

disubstituted dyads 9e and 9f. On the basis of calculated dipole to the charge transfer and the associated larger . Similarly, in

moments, notations are assigned to these dyads: d-A-a (5); D-s-A-a disubstituted analogues 9e and 9f, the observed trend in the

9a); D-s-A-A’ (9b); D-s-A-A” (9c); D-l-A-d (9d); D-s-A-s-D (9e) and D- values: 9f>9e (Table 3) is attributable to the increase in the π-

(

9

β

(

l-A-l-D (9f), where d-weak donor (Br); a-weak acceptor (CHO); D-

strong donor (Fc); A-strong acceptor (central unit); s-short

conjugated bridge; l-long conjugated bridge and A’ or A”-

Table 3. Quadratic nonlinear optical parameters of 5 & 9a-9f.

a

b

Compound

β_H_R_S

β_H_R_S_,₀

dicyanovinyl acceptors.

β values are in agreement with the dipole

-

30

-30

moments except 9b and 9c.

(10 esu)

Quadratic hyperpolarizability

5

68 ± 4

31 ± 2

104 ± 4

170 ± 7

502 ± 6

81 ± 4

77 ± 5

82 ± 4

The second-order nonlinear polarizabilities,

β, of 9a-9f were

-5

-4

measured for dilution series (10 -10 M) in THF at 840 nm using

9

a

207 ± 8

303 ± 13

913 ± 30

173 ± 8

152 ± 9

160 ± 8

5

9-64

HRS method

under ambient conditions. The octopolar

of the reference crystal violet was appropriately

2

7-28,87

symmetry

9b

taken care of and the difference in solvent was corrected for by the

optical local field correction factors. A multiphoton fluorescence

discrimination technique in the frequency domain has been applied.

Only for the DPP core 5, multiphoton fluorescence at 420 nm was

contributing to the scattering signal. With the high frequency

demodulation technique, it was possible to obtain an accurate

fluorescence-free second-order nonlinear polarizability value. The

concentration range used for the nonlinear experiments was small

enough to preclude any aggregation effects. The photostability

9

c

d

e

f

a

second-order nonlinear polarizability,

β

HRS, recorded at 840 nm in

THF. second-order nonlinear polarizability corrected for resonance

enhancement,

(under femtosecond pulsed 840 nm laser light) was checked by

b

comparing absorbance before and after the nonlinear experiment

and no differences were observed.

β

HRS,0

.

The principle factors, which determine the degree of polarization

conjugation bridge. Further, 9e and 9f were expected to have zero

dipole moments but the optimized geometries (Figure S21)

revealed slight distortion of the molecular planarity, leading to non-

are strength of donor and acceptor as well as the intervening π-

2, 12, 27, 88-94

conjugation bridge.

According to the two-level model

9

5-96

(equation 1)

:

zero dipole moments and hence non-zero β. Also, 9f has higher

oscillator strength (f) of CT band as well as lower band gap as

2

)

β

α ∆µge

r

ge / (Ege

--- (1)

Where,

β = second-order nonlinear polarizability.

compared to 9e, which may also be an additional contributory

factor in the higher

β value in the former. However, 9d shows

∆

µ

ge= difference in excited state and ground state dipole moments.

ge = transition dipole moment, which can be directly correlated to

oscillator strength (f) or molar extinction coefficient (∈).

ge = LE CT transition band gap.

Dyad 9c shows a greater

compared to the central DPP core 5. The order of the

higher than both 9e and 9f (Table 3), irrespective of its higher

β

r

band gap (Table 1) and lower oscillator strength (f) of CT band (See

SI Table S1). It could be explained on the basis of increased dipole

moment (Table 1) in this dipolar congener, leading to increase in

E

charge transfer and hence higher β.

β

value as compared to 9a and 9b

β

values for CONCLUSIONS

the dyads is: 9c> 9b> 9a (Table 3), although, 9b is appended with a

stronger dicyanovinyl electron acceptor compared to 9c as inferred

from the trend (9b>9c>9a, Table 1) of the calculated (TD-DFT:

B3LYP/6-31G) dipole moments in DCM medium. As the acceptor

strength increases from 9a→9b, whereas, the calculated band gap

decreases (Table 1), the oscillator strength (f) of CT band shows an

increasing trend (See SI Table S1), which could account for the

Synthesis of new ferrocene-DPP dyads has been described.

Femtosecond HRS measurements were performed at 840 nm

using a commercial Ti: sapphire laser at ambient temperature

and

revealed

structure

dependent

high

quadratic

hyperpolarizabilities. As deduced from the UV-visible absorption,

electrochemical, theoretical calculations, the second-order

nonlinear polarizability,

β, increased on increasing both, the

observed trend (9b>9a) of the

β values (Table 3). However, 9c

strength of acceptor as well as the length of the intervening π-

shows exceptionally high (Table 3) as compared to 9b, although

β

J. Name., 2013, 00, 1-3 | 13

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ARTICLE

conjugated linker, although the effect of former was more

Journal Name

11. K. Clays, Chem. Mater., 2003, 15, 642-648.

23

pronounced. This is in contrast to our earlier finding, wherein the

-values of ferrocene dyads were significantly modulated upon

1

2. M. G. Kuzyk, J. Mater. Chem., 2009, 19, 7444–7465.

β

increasing the length of the π-conjugated chain connecting the

ferrocene donor with an acceptor. Also, the LE CT bands of the

dipolar (D-A) dyads D-s-A-a (9a); D-s-A-A’ (9b); D-s-A-A” (9c), where

a-weak acceptor (CHO); D-strong donor (Fc); A-strong acceptor

1

3. S. R. Marder, D. N. Beratan and L.-T. Cheng, Science, 1991,

252, 103-106.

1

2

4. M. Lamrani, R. Hamasaki, M. Mitsuishi, T. Miyashitab and

Y. Yamamoto, Chem. Commun., 2000, 1595–1596.

(central unit); s-short conjugated bridge; l-long conjugated bridge

and A’ or A”-dicyaovinyl acceptors, appeared at lower energy as

well as showed smaller HOMO-LUMO band gaps as compared to

the disubstituted dyads, D-s-A-s-D (9e) and D-l-A-l-D (9f). Further,

these dyads showed negative solvatochromism. Further, the cyclic

voltammograms of the dipolar dyads 9b and 9c showed only one

reversible oxidation peak corresponding to the one-electron

oxidation of the Fc unit. However, the oxidation peaks of the

disubstituted dyads 9e and 9f observed a cathodic shift as well as

possessed roughly doubled amplitude due to two Fc units,

compared to that of the dipolar dyad 9a. Additionally, the

disubstituted dyads showed non-zero dipole moments, indicating

their non-centrosymmetric nature and hence non-zero quadratic

hyperpolarizabilities. A good correlation of the structural changes of

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the dyads as well as effect of substituents with the experimental β-

values was established. From the point of view of the development

of elctrooptic devices and applications, these new, thermally stable

DPP based dyads represent a good strategy for further exploration.

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Acknowledgements

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Synthesis, linear and nonlinear optical properties of thermally stable ferrocene-diketopyrrolopyrrole dyads

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