ChiralChiral ElectroanalysisElectroanalysis

Grants no. 2011-0417

(inherently chiral electrodes)

and no. 2011-1851

(inherently chiral ionic liquids) Serena Arnaboldi

1. Towards artificial enantioselective electrodes

Kuhn et al., Nature Comm. 2014

Attard, Feliu et al., Langmuir 1999

Torsi et al. Nature Materials, 2008

The development of artificial "intelligent" electrodes, capable to discriminate and quantify the enantiomers of chiralanalytes, particularly of biological and pharmaceutical interest,

is a quite attractive issue in electroanalysis.

Obviously, selectivity towards specular molecules can only be achieved in an enantiopure environment.

For this aim, many approaches have been proposed in the last years.

Towards artificial enantioselective electrodes

Aramata et al.,Chem. Commun. 1997

Nakanishi, Osaka et et al.,JACS Comm. 2006

Electrodes modified with chiral SAMs

Some examples of the proposed approaches I

Chiral monolayer of self-assembled ∆-[Os(bpy)2L(Cl)]+ [bpy = 2,2’-bipyridyl,

L = 1,2-bis(4-pyridyl)ethane] on a platinum electrode

Fu et al.,Electroanal. 2012

Enantioselective Recognition of Dopa Enantiomers

in the Presence of Ascorbic Acid or Tyrosine

Enantioselectivity of Redox Reaction of DOPA at the Gold Electrodes Modified

with a Self-Assembled Monolayer of Homocysteine

Marx et al., Langmuir, 2005

Advincula et al., Small, 2012

Electrodes modified with molecularly imprinted molecular layers

Some examples of the proposed approaches II

Chiral Electrochemical Recognition by Very Thin Molecularly

Imprinted Sol-Gel Films Nanostructured, Molecularly Imprinted, and Template-Patterned

Polythiophenes for Chiral Sensing and Differentiation

Molecularly imprintedchiral mesoporous Pt

Kuhn et al., Nature Comm. 2014Mogi & Watanabe, Sci. Tecn.

Adv. Mat. 2006

Electrodesmodified withasymmetricallygrownmolecularlayers bymagnetopolarization

Oxidation of organicmolecules on inherently

chiral metal surfaces

Attard, Feliu et al., Langmuir1999 & other papers in the

following years

Some examples of the proposed approaches III

Chiral organic thin-film transistor (OTFT)

A sensitivity-enhanced field-

effect chiral sensor

Torsi et al. Nature Materials, 2008

However, • even the most successful attempts at chiral discrimination almost invariably resulted in the detection of a difference in current intensity between the signals of the two antipodes of a chiral probe• the chiral enantioselective layer is in many instances not of general use, but tailored for a given probe;• many preparation procedures are very sophisticated/expensive…• … and/or the active films fragile.

Desirable features: • both peak potential separation and current linear dynamic range• easy, fast and low-cost preparation• equal availability in both enantiomer configurations• general applicability to many probes• reproducibility and stability• possibility of recycling the active surface• should work on different supports and in different operating media

A target still to be fully achieved

2. Our approach: inherently chiral electroactive oligomer filmsfrom inherently chiral heterocycle-based monomers

Intrinsic 3D character

Intrinsic 3D character

Intrinsic regioregularity in

polymerization

Intrinsic regioregularity in

polymerization

Inherent dissymmetry:The whole molecule is chiral, exhibiting a C2 symmetry axis.

Energy barrier is sufficiently high to yield stable

enantiomers.

Inherent dissymmetry:The whole molecule is chiral, exhibiting a C2 symmetry axis.

Energy barrier is sufficiently high to yield stable

enantiomers.

S

S

S

S

The first example: BT2T4 and oligo-BT2T4 films

F. Sannicolò, S. Arnaboldi et al. Angewandte Chemie Int. Ed., 2014, 53, 2623S. Arnaboldi et al. Chemical Science, 2015, 6, 1706

2,2’-bis(2,2’-bithiophene-5-yl)-3,3’-bi-1-benzothiophene

SS

S S S

S

70°, ~50 kcal mol-1

Pt electrode,

CH2Cl2+0.1 M TBAPF6,

0.2 V s-1

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

0 0.25 0.5 0.75 1 1.25 1.5

E / V(SCE)

i/c

/ (

A m

ol

-1 d

m3 c

m-2

)

I

VI

XII

XVIII

XXIV

XXXI

XXXV

0.0005 M BT2T4

Pt foil electrode, CH2Cl2 + 0.1 M

TBAPF6, 0.2 V s-1

Electrooligomerization of the BT2T4 monomer

Fast and regular film electrodeposition in a widerange of conditions, even at low monomer

concentration

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1 1.25

E /V (Fc+/Fc)

I / m

A

-1100

-1000

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

ηf / H

z

0

0.0000005

0.000001

0.0000015

0.000002

0.0000025

0.000003

0.0000035

0.000004

0 25 50 75 100 125 150 175 200 225

t /s

ηm/g

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

monomer u

nits n

m-2

Au electrode (EQCM), CH2Cl2 + 0.1 M TBAPF6, 0.2 V s-1

0.001 M BT2T4

Regular increa

se of the

polymer film

Counteranioningress/egress

SS

SS

S

S SSS

SSS

S S S

S S S

n-1BT2-T4

(BT2-T4)n n = 2-5

oxidation

S S S

S S S

SSS

SSS

Electrooligomerization yields cyclic oligomers!

Dimer

Recently, we found byhigh resolution MALDI that the electrodepositedfilms mostly consist of cyclic oligothiophenes, constituted by 12, 18, 24 … conjugated thiopheneunits!

Electrochemical or chemical oxidation

(by FeCl3)

The same cycles alsoconstitute a large fraction of the electrodepositedoligomer films.

The cyclic vs linearelectrodeposited oligomerratio appears to depend on the electrode surfacematerial (GC>>ITO)

F. Sannicolò, S. Arnaboldi et al., Chemistry 2014F. Sannicolò, S. Arnaboldi et al., Pat. Appl., MI2014A000948

Trimer

SSS S

S

S

S

S

S S

S

S

S

S

S

S

S

S

The new molecular materials possess an outstanding pool of attractive properties.

Even as racemates:

Oligomer properties as racemates

SSS

SSS

S S S

S S S

n-1

(BT2-T4)n n = 2-5

they provide cavitiesfunctionalized withheteroatoms, which, like e.g cyclodextrins, can act as hosts for a variety of guests

they idealize conducting polymerswithout end, with no defectivityconnected with active terminals

they exhibit very facile, reversible charge

transfer and very fast charge transport, as

revealed by CV and EIS

their redox potentials are convenient for energy

applications, and modulable by structure

design

they are electrochromic,photoactive, and display charge-

trapping effects

an appropriate protocolaffords the oligomer films tobe obtained as self-standing membranes

However, chirality can make them even smarter!

Elution time (min)

Signal(m

V)

(S,S)-(-)-2

(R,R)-(+)-2

(R,R-S,S)-(±)-2

6 8 10 12

0

1

2

-9

-6

-3

0

0

13

26

39

Enantiopure inherently chiral electroactive macrocycles

Starting from enantiopure monomers,

enantiopure inherently chiral macrocycles are obtained

M.C. Escher

Elution time (min)10 12 14 16 18

0

1

2

-4

-2

0

0

20

40

60

(R,R,R)-(-)-3

(S,S,S)-(+)-3

(R,R,R-S,S,S)-3

Signal(m

V)

Elution time (min)10 12 14 16 18

0

1

2

-4

-2

0

0

20

40

60

(R,R,R)-(-)-3

(S,S,S)-(+)-3

(R,R,R-S,S,S)-3

Signal(m

V)

Chirality makes the ringlet films even smarter, endowing them with an exceptionalseries of additional properties!

Oligomer properties as enantiopure antipodes

Impressive polarized light rotation angles ([α]D

25∼2000 for the trimer)

Sharp circular dichroism spectra

outstandingenantiodiscrimination ability

as chiral electrodes

Circularly polarizedluminescence

New!! Self-supportedelectroactivechiral membranes!

CD

(m

deg

)

New!! Self-supportedelectroactivechiral membranes!

300 350 400 450 500

-60

0

60

mdeg

nm

CD

(m

deg

)

-12

0

12

300 400 500 600 700 800 900

nm

mdeg

CD

(m

deg

)

Conformational features and chirality manifestations reversibly

modulable by the electricpotential

Monomer

Oligomer

3. Enantiorecognition tests

Potential-Driven Chirality Manifestations and ImpressiveEnantioselectivity by Inherently Chiral Electroactive Organic Films

Angew. Chem. Int. Ed. 2014, 53, 2623 –262

Inherently chiral electrodes: the tool for chiral voltammetry, Chemical Science 6 (2015) 1706

First enantiorecognition tests (I)

Neat separation between the twoenantiomers (∼100 mV)!

R

FeFe

S

H

Me

NMe2H

Me

Me2N

(S)-(−−−−)-N,N-

dimethyl-1-ferrocenylethylamine

(R)-(++++)-N,N-dimethyl-1-ferrocenylethylamine

(Having identical CV pattern on bare Au SPE)

electrooligomerizationat low scan rate (0.05 V/s) in BMIMPF6 ionic liquid,

on Au screen printed electrodes(36 cycles, 0.012 M)

Oligo-(S)-(+)-BT2T4

Oligo-(R)-(−−−−)-BT2T4

(R)-(−−−−)-BT2T4 (S)-(+)-BT2T4

diastereomeric redox couples:

∆E’ ≈ 100 mV

enantiomer redox couples: ∆E°’ ≈ 0 mV

S S R R

RS R S

R

FeFe

S

H

Me

NMe2H

Me

Me2N

Inserting the probe racemate,

two neatly separatedpeaks are observed(e.e. determination)

First enantiorecognition tests (II): Racemates and enantiomeric excesses

Oligo-(S)-(+)-BT2T4

Sprobe

S film

R probe

Energetics of a binary system can be significantly differentfrom that of a ternary system

H. Wynberg and B. Feringa, Tetrahedron, 1976, 32, 2831

Enantiorecognition tests with a single ringlet kind

Drop coating of enantiopure ringletsolutions in CH2Cl2

on Au SPEs,

then tests in BMIMPF6

4. Confirming the enantiorecognition capabilitywith different probes

-0.003

-0.002

-0.001

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

j / (A cm -2)

E vs Fc+|Fc / (V)

0 M

0.001 M

0.002 M

0.003 M

0.005 M

0.0065 M

0.0075 M

0.0085 M

0.0100 M

0.00000

0.00005

0.00010

0.00015

0.00020

0.000 0.002 0.004 0.006 0.008 0.010 0.012

I / (A)

clevo-ofloxacin / (mol dm-3)

The electrodesurface responsedisplays a linear

dynamic rangeChiral Probe:

(S)-ofloxacin

-0.001

0.000

0.001

0.002

0.003

0.004

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

j/ (A cm -2)

E vs Fc+|Fc / (V)

(S)-(T2BT)2

(R)-(T2BT)2

Chiral Probe:

(S)-ofloxacin

(R)-(-)-BT2T4

(S)-(+)-BT2T4

Tests on GC supportin ACN+TBAPF6

Enantiorecognition Capability using chiral probes with different bulkiness and chemical nature

N

F

N

N O

O

OH

O

HMeMe

N

F

N

NO

O

OH

O

HMe Me

(S)-ofloxacin (R)-ofloxacin

Enantiorecognition tests towards DOPA probe

D-DOPA and L-DOPA signals

on bare GC electrode

-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

j / (Acm-2)

E vs Fc+|Fc / (V)

L-DOPA

D-DOPA

(R)-(BT2 -T4 )n

-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

j / (A cm-2)

E vs Fc+|Fc (V)

L-DOPA

D-DOPA

(S)-(BT2 -T4)n

(R)-BT2T4

(S)-BT2T4

Tests alternating D- and L-

DOPA on both

enantiopurefilms

Tests on GC support

in H2O+HCl 0.05M

-0.0010

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

j / (A cm -2)

E vs Fc+|Fc/ (V)

L-DOPA

D-DOPA

OH

OH

OH

O

NH2

OH

OH

OH

O

NH2

(R)-BT2T4(S)-BT2T4

OH

OH

OH

O

NH2

M.C. EscherBond of Union

Enantiorecognition tests towards Naproxen, Catechin, Norepinephrine, Ascorbic Acid

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

1.4E-04

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

E vs Fc+|Fc (V)

j / (A cm-2)

(S)-BT2T4

(R)-BT2T4

Naproxen

Buffer Solution pH7

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

1.4E-04

1.6E-04

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

E vs Fc+|Fc (V)

j/ (A cm-2)

(S)-BT2T4

(R)-BT2T4

(−−−− )-CatechinBuffer solution pH 7

-0.0004

-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

E vs Fc+|Fc (V)

j / (A cm-2)

(S)-BT2T4

(R)-BT2T4

L-(−)-Norepinephrine (+)-bitartrate

salt monohydrate

HCl Aqueous Solution (0.05 M)

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

E vs Fc+|Fc (V)

j / (A cm-2)

(S)-BT2T4

(R)-BT2T4

L-Ascorbic Acid

ACN+TBAPF6

5. Confirming the conceptwith chemically different starting monomers

NMe N

Me

S

SS

SS S

SS

S

S

S

S

Enantiorecognition tests using pyrrole-based atropisomeric scaffold

Enantiorecognitiontests on GC electrode in a 0.05 M HClaqueous solution

-1.0E-03

0.0E+00

1.0E-03

2.0E-03

3.0E-03

4.0E-03

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

j / (A cm-2)

E vs Fc+|Fc / (V)

L DOPA

D DOPA

2,2’-Ind2T4(N-Me)-1Electrooligomerizationon GC electrode in

DCM+TBAPF6 0.1M

•100o node angle between moieties

•Possibility to locate the heteroatom in ortho position

•Two homotopic positions available forelectrooligomerization

NMe N

Me

S

SS

S

OH

OH

OH

O

NH2

OH

OH

OH

O

NH2

OH

OH

OH

O

NH2

OH

OH

OH

O

NH2

SS

S S

S S

SS

T84

The same concept in all-thiophene materials: inherently chiral spider-like oligothiophenes

α, α-link: a node/distortionarises, but the energy

barrier for rotation is low

S

SS

S

S

S

S

S

S

SS

S

S

S

S

S

β, β-link: a node/distortion withhigh energy barrier fo rotation

→→→→ the molecule is chiral

α, α-link: a node/distortionarises, but the energy

barrier for rotation is low

β, β-link: a node/distortion withhigh energy barrier fo rotation

→→→→ the molecule is chiral

α, α-link: a node/distortionarises, but the energy

barrier for rotation is low

β, β-link: a node/distortion withhigh energy barrier fo rotation

→→→→ the molecule is chiral

M. C. Escher

-1500

-1000

-500

0

500

1000

1500

2000

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5

j/ (µA cm

-2)

E vsAg/AgCl /V

oligo-T83-1electrode

(R)(S)

-1500

-1000

-500

0

500

1000

1500

2000

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5

j/ (µA cm

-2)

E vsAg/AgCl /V

oligo-T83-2 electrode

(R) (S)

-2000

-1000

0

1000

2000

3000

4000

5000

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

j/ (µA cm

-2)

E vsAg/AgCl /V

oligo-T83-1 electrode

((((±±±±))))

Similar tests on inherently chiral spider-like oligothiophenes(ββββ,ββββ’-bithiophene core)

R

FeFe

S

H

Me

NMe2H

Me

Me2N

-1300

-800

-300

200

700

1200

0.2 0.3 0.4 0.5 0.6 0.7

j/ (µA cm

-2)

E vsAg/AgCl /V

oligo-T83-1electrode

D-DOPA

L-DOPA

L/D-DOPA on bare Au electrode

OH

OH

OH

O

NH2

OH

OH

OH

O

NH2

Enantiorecognition tests on SPEs both in BMIMPF6 and in

aqueous HCl

highly 3D,

four homotopic

αααα positions foroligomerization,

but does not give

cyclic oligomers

• both peak potential separation and current linear dynamic range

Coming back to the checklist of desiderable features…

Conclusions 1

• reproducibility and stability

• general applicability to many probes

• equal availability in both enantiomer configurations

• easy, fast and low-cost preparation

• efficiency on different supports and operating media

• possibility of recycling the active surface

COMPLIES

COMPLIES

COMPLIES

COMPLIES

COMPLIES

COMPLIES

COMPLIES

Moreover, •The concept works as well in chemically different oligomers•The enantiopure film can be also obtained and processed as self-standing membranes

Inherently chiral electroactive films are indeed attractive tools for chiral voltammetry

6. An alternative approach:Working on an achiral surface

but in an inherently chiral medium

An alternative approach to chiral electroanalysis: chiral working media

As an alternative approach to using a chiral electrode surface, differentchiral media for electrochemical processes have been proposed

Chiral organic solvents

Chiral supportingelectrolytes

Increasingly more orderedat the electrode/solutioninterphase, resulting in increasing enantioselectiveeffects

Chiral ionic liquids(CILs)

Already adopted in organic chemistry, still to be explored in electrochemistry

Possibly the best:

Inherently chiral ionic liquids(ICILs)

The cationic bibenzimidazolium or bipyridinium moiety responsible for the CILs physical properties is also part of the stereogenic element responsible for

molecular chirality.

Inherently Chiral Ionic Liquids

N

N

N

R

N

R

N N

NNR

R

N

R

RR

N

RR

R

+N

N

N

N

R'

R' +X

X

R

R

+NN

NN

R

R

+

X

X

R'

R'

N

R

N

R

R'

R'

RR

+

+

X-

X-

3,3’-Bi(collidinium) cation

220 240 260 280 300 320 340

-40

-30

-20

-10

0

10

20

30

40

Ellipticity (mdeg)

Wavelength (nm)

Firs t Eluted

Se cond Eluted

Mono- and di-alkylation

Easy synthesis Scalable procedures

N Me

MeN

Me

Me

Me

Me

MeMeSO3

N Me

MeN

Me

Me

Me

Me

MeMeSO3

MeMeSO3

N Me

MeN

Me

Me

Me

Me

CH2PhMeSO3

N Me

MeN

Me

Me

Me

Me

Ch2PhMeSO3

CH2PhMeSO3

6 8 10

0

200

400

600

6 8 10

0

20

40

6 8 10

0

50

100

150

200

CSP: Chiralpak IA-3 250 mm x 4.6 mm i.d.

Mobile phase: n-hexane-ethanol-DEA 100:2:0.3Flow rate: 1 mL/min

Detector: UV (black) and CD (red) at 254 nm

Temperature: 15 °C

DFT calculation demostrated that the racemisation

barrier of the

3,3’-bicollidine is 42.5 kcal mol-1.

The most promising family: bicollidinium scaffolds and salts

∼ 3.6 V potential window

220 240 260 280 300 320 340

-40

-30

-20

-10

0

10

20

30

40

Ellipticity (mdeg)

Wavelength (nm)

First Eluted

Second Eluted

First inherently

chiral ionic liquids at

room temperature!!

∼ 3.6 V potential window

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

E vs Ag|Ag+ / V

j/c / Acm

-2mol-1dm

3

(R)-Fc

(S)-Fc

(R)-Fc and (S)-Fc on bare Electrode

(1)-3mE2BF4 0.01 mol dm-3 in BMIMPF6

(2)-3mE2BF4 0.01 mol dm-3 in BMIMPF6

(R )-Fc e (S )-Fc 0.002 mol dm-3

N

Me

Me Me

N

Me Me

Me

Et

Et

BF-4

BF-4

Ratio 1 : 500Ratio 1 : 500

Recently we have confirmed that also these materials hold an impressive enantiorecognition ability like a

chiralizing gel spread on electrodes.

Breaking news: enantiorecognition tests employing bicollidinium salts!!!

N

Me

Me Me

N

Me Me

Me

Et

Et

BF-4

BF-4

NN

MePF-

6

NN

MePF-

6

NN

MePF-

6

NN

MePF-

6

NN

MePF-

6

NN

MePF-

6

NN

MePF-

6

NN

MePF-

6

NN

MePF-

6

The support of grants no. 2011-0417 (inherently chiral electrodes) and no. 2011-1851 (inherently chiral ionic liquids) is gratefully acknowledged

The Milano

researchers are

members

of the

newly set up

“SmartMatLab

Laboratory

and Training

Centre”

offering

state-of-the-art

facilities

which will support

the preapplicative

optimization

of these

“intelligent”

electrode surfaces