Bright Electrochemiluminescence Tunable in the Near-Infrared of

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Bright Electrochemiluminescence Tunable in the NearInfrared of Chiral Cationic Helicene Chromophores Haidong Li, Antoine Wallabregue, Catherine Adam, Maria Géraldine Beltran Labrador, Johann Bosson, Laurent Bouffier, Jerome Lacour, and Neso Sojic J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11831 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 17, 2016

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The Journal of Physical Chemistry

Bright Electrochemiluminescence Tunable in the Near-Infrared of Chiral Cationic Helicene Chromophores

Haidong Li,a Antoine Wallabregue,b Catherine Adam,a Geraldine M. Labrador,b Johann Bosson,b Laurent Bouffier,a Jérôme Lacour,*,b Neso Sojic*,a

a

Univ. Bordeaux, INP Bordeaux, ISM, UMR CNRS 5255, 33607 Pessac,

France. E-mail: [email protected]

b

Department of Organic Chemistry, University of Geneva, Quai Ernest

Ansermet 30, 1211 Geneva 4, Switzerland. E-mail: [email protected]

Abstract Cationic helicenes are ortho-fused polyaromatics which exhibit well-defined and stable helical conformations with original absorption and emission properties in the red to near-infrared spectral ranges. Herein, a selection of cationic [4] and [6]helicenes

are

studied

for

their

electrochemical,

fluorescence

and 1

ACS Paragon Plus Environment


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electrochemiluminescence

(ECL)

properties

in

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acetonitrile

solutions.

Their

photophysical and redox responses are drastically tuned by the introduction of auxochrome substituents at their periphery or the interconversion of oxygen and nitrogen atoms within the helical core. All diaza helicenes exhibit a reversible reduction process whereas, in the presence of oxygen instead of nitrogen atoms in the helical core, irreversible oxidations and a decrease of ECL intensity are observed. ECL emission was successfully produced with two sacrificial co-reactants (benzoyl peroxide

and

tri-n-propylamine,

TPrA).

[4]Helicene

DMQA+,

[6]helicene

DIAZA(Pr/Br)+ and DIAZA(Hex/Br)+ exhibit similar ECL emission wavelength in the near-infrared region and generates very intense ECL signals. Their ECL efficiencies are up to 2.6 times higher than that of the reference compound [Ru(bpy)3]2+ when using TPrA as co-reactant. A thermodynamic map gathering both oxidation and reduction potentials and fluorescence data is proposed for the prediction of energy sufficiency needed in both co-reactant ECL systems. Such a systematic overview based on the photophysical and electrochemical properties may guide the conception and synthesis of new chromophores with a strong ECL proficiency.

Introduction Helicenes are polyaromatic derivatives displaying a helical conformation due to the presence of sequential ortho-fused benzenes or other hetero-aromatic rings. A large variety of such moieties are available today with applications ranging from chirality studies to catalysis, optoelectronic and biology.1-8 Within this family of compounds, cationic [4] and [6]helicenes 1-7 (Figure 1)

9,10-26

that are related to

aza/oxa triangulenes,10 stand apart as being effective organic chromophores in the red to far-red spectral range and, in terms of synthesis, are readily post-functionalized with auxochrome substituents at their periphery.17,

22-23

Such a modulation of the

chemical, electronic and optical properties of the dyes can also be achieved by the interchange of nitrogen atoms by oxygens within the helical core.18,

27-29

These 2


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combined synthetic possibilities that help fine-tune not only the absorption and emission of the dyes but also the electrochemical properties led us to study their behavior in electrochemiluminescence (ECL).

+

Figure 1. Cationic diaza [4]helicene 1 (DMQA : dimethoxyquinacridinium), diaza [6]helicene 2 + + + (DIAZA ), azaoxa [6]helicene 3 (AZAOXA ), dioxa [6]helicene 4 (DIOXA ) and functionalized diaza [6]helicenes 5-7. Only M-configurations shown.

ECL is a type of chemiluminescence involving the in-situ generation of an excited state at the electrode surface by electron transfer (ET) reactions.30-31 ECL mechanisms have been established in two dominant pathways. In the annihilation pathway, an oxidized and a reduced species of the chromophore are electrochemically produced at the electrode surface by applying alternating pulsed potentials.32 Then, these two species react together by an annihilation reaction to generate an electronically excited state. It relaxes to the ground state and emits a photon. An alternative mode of ECL generation with more complex kinetic pathways is based on the use of sacrificial co-reactants (e.g. tri-n-propylamine (TPrA), oxalate, benzoyl peroxide (BPO), etc.) which yield reactive species upon ET. The latter are then capable of reacting with the oxidized or reduced chromophore to generate the desired excited state. Since by essence ECL does not require any external light source for the excitation, it is a very sensitive technique which is based on orthogonal electrochemical

addressing

and

optical

detection

modalities.

Due

to

its

interdisciplinary nature and also to its remarkable characteristics, ECL has attracted large interests in different fields such as photochemistry, electrochemistry, analytical 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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chemistry, etc.32-35 ECL has shown great achievements in fundamental research but also for real applications, mainly in the healthcare diagnostic market with immunoassays.33 The model systems are mainly based on organic dyes such as aromatic heterocycles and organometallic compounds such as ruthenium and iridium complexes.35-44 To improve the analytical performances of ECL, considerable efforts are currently focused on the development of novel ECL-active dyes with higher efficiency and/or tunable wavelengths.36, 42, 45-48 In the present work, electrochemical and ECL properties of cationic helicene dyes 1-7 are reported. Cyclic voltammetry was used to investigate their electrochemical behavior in acetonitrile. The stability of the helicene radical ions was also compared to draw a structure/activity relationship depending on the nature of the heteroatoms included in the helical core as well as the pending substituents. The ECL emission was recorded with two co-reactants. The data for the ECL efficiency were collected by comparison with [Ru(bpy)3]2+ used as a standard under the same experimental conditions. Finally, the threshold energetic requirement for generating ECL49 in the helicene/co-reactant systems was displayed by using an ECL map covering both anodic and cathodic regions.

Experimental section Materials Cationic helicenes 1-5 and 7 were synthesized according to previously reported procedures.11-12,

17, 22

The preparation of DIAZA(Hex/Br)+ 6 is reported in the

supporting information (Figure S1).17 Other reagents were purchased from Sigma-Aldrich and used without purification unless otherwise stated. Instrumentation Fluorescence spectra were collected on a Varian Cary Eclipse spectrophotometer, 4


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using a 1 cm-path length quartz cuvette. Voltammetric experiments were performed with a PGSTAT30 Autolab potentiostat connected to a conventional three-electrode cell, consisting in a silver-wire pseudo-reference electrode, a platinum-wire auxiliary electrode, and a 1 mm in diameter platinum disk working electrode. Prior to measurements, Pt disks were polished with alumina slurry of different size, rinsed thoroughly with Milli-Q water between each polishing step, and sonicated in water and ethanol respectively, followed by a final rinse with acetonitrile and dried with N2 stream. ECL intensity was measured using a Hamamatsu photomultiplier tube R5070A with a Hamamatsu C9525 high voltage power supply. The PMT detector was held at – 750 V and placed a few millimeters in front of the working electrode. The output signal was amplified by a Keithley 6485 Picoammeter before acquisition via the second input channel of the PGSTAT30 Autolab potentiostat. ECL spectra were recorded using a Princeton Instruments Acton SpectraPro 2300i with a 1 mM solution of the helicene compound in acetonitrile with 0.1 M TBAPF6 as supporting electrolyte. The cell is built with a glass slide at the bottom in order to record the ECL signal. The optical fiber connected to the device is placed close to this glass slide in front of the working electrode. The potential is controlled by a potentiostat (µ-Autolab type III), the pseudo-reference electrode is a silver wire and the counter electrode is a platinum wire. Before each experiment, the solution is degassed using Argon or Nitrogen for 5 min. ECL was generated with 10 mM BPO by applying a constant potential of – 1.5 V vs. Ag. For experiments with TPrA, the concentration of the co-reactant was 50 mM and the working electrode is kept constant at + 1.5 V vs. Ag.

Results and discussion Electrochemistry The series of [6]helicenes 2-7 was investigated by recording cyclic 5



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voltammograms and by comparing the corresponding electrochemical behavior to that of classical DMQA+ 1 used as reference (Figure 2).19 DMQA+ 1 shows a relatively simple redox behavior. It is typically reduced at − 0.70 V vs. Fc/Fc+ with a peak to peak separation (∆Epeaks) of ~ 56 mV evidencing a reversible monoelectronic transfer (Table 1). The oxidation also shows a reversible behavior and is centered at 1.4 V vs. Fc/Fc+ generating a dicationic species from the initial cationic form. The voltammetric characteristics of DIAZA(Pr)+ 2 have been reported recently.22 Briefly, it shows very similar electrochemical features as 1, exhibiting also reversible redox monoelectronic processes at − 0.72 V and + 1.4 V vs. Fc/Fc+, respectively. Another irreversible process is also observed at more positive potentials and starts at about 2 V. This was assigned to an irreversible oxidative decomposition of the diaza[6]helicene core but it was not further investigated. The similar electrochemical behavior of DMQA+ 1 and DIAZA(Pr)+ 2 indicates that substituting methoxy groups in the [4]helicene family by additional benzene rings (i.e. [6]helicene) marginally affects the redox potentials thanks to comparable donating ability.

Figure 2. a) Influence of the nature of the heteroatoms in the helical core on the voltammetric behavior of helicenes: DMQA+ 1, DIAZA(Pr)+ 2, AZAOXA+ 3 and DIOXA+ 4. b) Influence of the peripheral substituents on the electrochemistry of DIAZA+ derivatives 5-7. Cyclic voltammograms of 10–3 M helicenes were recorded in degassed CH3CN solution containing 0.1 M TBAPF6 as supporting electrolyte. Scan rate 0.1 V.s–1.

6


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By contrast, interchanging N-atoms by oxygens in the cationic core drastically modified the corresponding voltammograms. Indeed, on one hand, by comparing DIAZA(Pr)+ 2 to DIOXA+ 4, the reductive behavior is apparently unchanged with a single reversible one-electron process, but occurring at a much milder cathodic potential. On the other hand, no reversible oxidation was observed with compound 4 with only a fully irreversible process occurring at large anodic overpotentials (2.14 V vs. Fc/Fc+). This directly highlights the structure/activity influence of the nature of the heteroatoms at bridge positions of the [6]helicenes as it strongly modulates the stabilization of a dicationic structure. Finally, the electrochemical characterization of the non-symmetrical AZAOXA+ derivative 3 reveals a strong analogy with the behavior of DIOXA+ (Ered = − 0.45 vs. Fc/Fc+ with ∆Epeaks ~ 58 mV, and Eox = 1.93 V vs. Fc/Fc+). The electrochemical mechanisms are perfectly comparable for AZAOXA+ 3 and DIOXA+ 4 demonstrating that the replacement of a single nitrogen by a O-atom perturbs irreversibly the oxidation process. As previously observed for cationic triangulene structures,9,

21

the replacement of N heteroatoms by oxygens

decreases the donor effect. Therefore the reduction becomes easier for AZAOXA+ 3 and DIOXA+ 4 compared to DMQA+ 1 and DIAZA(Pr)+ 2 and the voltammetric waves appear at less cathodic potential values (Figure 2a). Functional groups such as Br or NO2 can be introduced at the periphery of the diaza [6]helicene helical core. Cyclic voltammograms of compounds 5-7 were recorded to investigate the influence on the electrochemical behavior caused by such functionalizations. In a similar fashion to DIAZA(Pr)+ 2, compounds 5-7 displayed a reversible monoelectronic reduction at less cathodic potential (Table 1). The presence of bromine substituents at the periphery does not affect the reversibility of the oxidation process. However, the introduction of NO2 groups at the same respective positions strongly affects the oxidation behavior which becomes irreversible. In summary, the electrochemical investigation reveals that for all [6]helicenes a reversible monoelectronic reduction and an oxidation that could be either reversible (DIAZA+ 2 and 5-7) or irreversible (AZAOXA+ 3 and DIOXA+ 4) occur. Due to the donor effect, N heteroatoms within the helicene core (DMQA+ 1 and DIAZA(Pr)+ 2) 7



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also stabilize more efficiently the dicationic species generated during the oxidation process by comparison to the compounds containing O atoms (AZAOXA+ 3 and DIOXA+ 4). Therefore, the oxidation waves of these latter compounds are shifted to higher potentials and become irreversible. The fine tuning of the redox properties can also be achieved through a careful choice of the nature of chemical functions pending on the nitrogen heteroatoms or aromatic ring as exemplified in the DIAZA+ series (2, 5-7). Such diverse electrochemical features may lead to dissimilar ECL capability with both co-reactants because these [6]helicenes exhibit different potentials for the reduction and oxidation processes. Compound

Ered(a)

Eox (a)

E / V vs. Fc/Fc+ +

(d)

஻௉ை Fluo. ECL (b) ΦFluo (c) ߔா஼௅

λmax/nm

்௉௥஺ ߔா஼௅

%

%

%

46.2

231

37.9

100

‒ 0.70

1.40

670

670

‒ 0.72

1.40

657

664

13 52.3

DIAZA(Pr/Br) 5

‒ 0.38

1.56

680

686

5.6

8.3

236

DIAZA(Hex/Br)+ 6

‒ 0.43

1.51

682

689

6.8 83.6 35.8 16.6

12.5

264

DMQA 1 +

DIAZA(Pr) 2 +

+

DIAZA(Pr/NO2) 7 +

AZAOXA 3 +

DIOXA 4

‒ 0.30

-

600

-

‒ 0.45

-

616

616

‒ 0.12

-

592

602

-4

Bright Electrochemiluminescence Tunable in the Near-Infrared of

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