Electrochemistry Communications 6 (2004) 325–330

www.elsevier.com/locate/elecom

Electrochemistry coupled to fluorescence spectroscopy:a new versatile approach

Maryl�ene Dias a, Pietrick Hudhomme b, Eric Levillain a,*, Lara Perrin b,Yucel Sahin c, Franc�ois-Xavier Sauvage d,*, Corinne Wartelle d

a Groupe Syst�emes Conjugu�es Lin�eaires, CIMMA, UMR CNRS 6200, 2 Boulevard Lavoisier, F-49045 Angers Cedex, Franceb CIMMA, UMR CNRS 6200, 2 Boulevard Lavoisier, F-49045 Angers Cedex, France

c Department of Chemistry, Anadolu University, T-26470 Eskis�ehir, Turkeyd LASIR-HEI, UMR CNRS 8516, 13 rue de Toul, F-59046 Lille Cedex, France

Received 18 December 2003; received in revised form 14 January 2004; accepted 14 January 2004

Published online: 3 February 2004

Abstract

The efficiency and versatility of a spectroelectrochemical cell was used in order to couple electrochemistry and fluorescence

spectroscopy. This new tool was tested and validated through the study of the reduction steps of a soluble perylene derivative.

Besides the establishment of the method, the efficiency of UV–Vis spectroelectrochemistry and fluorescence spectroelectrochemistry

were compared.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Electrochemistry; Fluorescence spectroscopy; Spectroelectrochemistry; Perylene diimide

1. Introduction

Redox reactions can be carried out in bulk solution,

but this method provides very little control over the

processes taking place. Electrochemistry brings a much

cleaner way to perform redox reactions. By controlling

the potential applied to a working electrode (i.e., during

a cyclic voltammetric experiment), it is possible to

monitor the electron transfers occurring and investigatethe kinetics of the reaction mechanisms involved. Fur-

thermore, the electrochemical cell can be designed in

such a way that the side products will not interfere with

the processes being studied. On the other hand, elec-

trochemistry brings very little information concerning

the structural changes taking place in the molecules

being handled. Spectroelectrochemistry is the coupling

of an electrochemistry experiment with a spectroscopic

* Corresponding authors. Tel.: +33-2-41-73-50-95; fax: +33-2-41-73-

54-05 (E. Levillain).

E-mail addresses: eric.levillain@univ-angers.fr (E. Levillain),

fxsauvage@hei.fr (F.-X. Sauvage).

1388-2481/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2004.01.010

technique. If the two experiments are carried out in thesame cell and simultaneously, it is referred to as ‘‘in situ,

real time’’. Such an experimental setup allows to corre-

late the electron transfers to the spectroscopic events

taking place during the redox reaction.

Within this framework, a few years ago a multipur-

pose spectroelectrochemical cell was designed and built

in our lab [1]. This cell was inspired by Salbeck�s work[2]. The original idea was to be able to adapt the samecell over a broad series of spectroscopic techniques: the

main body of the cell and electrodes would remain

identical from one spectroscopic technique to another,

while a specific adapter would be added for each kind of

spectroscopy. It was first used in the UV–Vis range. In

this case, the light was brought to and collected from the

cell by fiber optics. The cell also found applications with

Raman detection [3]. More recently, its use was ex-tended to the infrared region. In this latter case, the

design of the cell was improved and its reliability was

ascertained with the well known model system TCNQ

(tetracyanoquinodimethane) [4]. Later on, the cell

proved to be a unique and very reliable tool for the

326 M. Dias et al. / Electrochemistry Communications 6 (2004) 325–330

study of the oxidation of TTF (tetrathiafulvalene) andone of its derivatives, TMT-TTF (tetramethylthio-tet-

rathiafulvalene) [5].

Considering the high sensitivity of fluorescence and

the salient structural information it can provide [6], it

was found interesting to adapt our cell to this spectro-

scopic technique. Publications devoted to spectroelec-

trochemistry coupled to fluorescence are scarce in the

literature. Some papers have been published on in situfluorescence spectroelectrochemistry, by using either li-

quid–liquid interfaces under controlled potential [7,8],

or a detachable rotating disc in order to measure in situ

fluorescence on redox active films [9], or in situ fluo-

rescence microscopy [10], or reporting spectrofluoro-

metric hydrodynamic voltammetry [11]. Other previous

papers used an OTTLE (optically transparent thin layer

electrode) [12–14]. Recently, a full study used evanescentwave techniques in order to measure in situ fluorescence.

It was based on a novel spectroelectrochemical sensor

that incorporates multiple internal reflection spectros-

copy at an optically transparent electrode coated with a

selective film in order to enhance detection limits by pre-

concentrating the sample at the OTE surface [15].

This paper reports the validation of a new versatile

approach to couple electrochemistry and fluorescence byadapting our spectroelectrochemical cell in order to

avoid the use of OTTLE and comply with a spectro-

fluorimeter.

A strong fluorescent molecule that undergoes a fully

reversible electrochemical process, i.e., a soluble deriv-

ative perylene diimide, has been used to establish the

method (Chart 1). Perylene dyes have been known since

1913 as highly photostable pigments or vat dyes. Per-ylene diimides have been studied because of their bril-

liant color, strong absorption and fluorescence, as well

as good thermal, chemical, and photochemical stability

[16–23]. Furthermore, they undergo fully reversible

electrochemical processes in aprotic media [24].

NN

OO

O O

C5H11C5H11

Cl

Cl

Cl

Cl

Chart 1. Perylene dipentylimide (1).

OO

OO

O O

N

O

O

C5H11

C5H11-NH2,∆ DMF

quantitative yield

Scheme 1. Synthesis of pery

2. Experimental

Perylene dipentylimide (1) was prepared in two steps

(Scheme 1) by using successively the condensation of

pentylamine on perylene 3,4,9,10-tetracarboxylic dian-

hydride, according to a reported procedure [25], fol-

lowed by a reaction with sulphuryl chloride in the

presence of iodobenzene and iodine as catalysts [26].

All spectroscopic data (1H, MS, IR) were in agree-ment with the structure described for 1: 1H NMR

(CDCl3) d (ppm): 8.70 (s, 4H), 4.20 (t, 4H), 1.80 (m,

4H), 1.40 (m, 8H), 0.95 (t, 6H); MS (DCIþ-CH4): 668

(Mþ:); IR (KBr): 1705 cm�1, 1665 cm�1 (C@O); UV–Vis

(CH2Cl2): kmax ¼ 425, 480, 520 nm (e ¼ 8:9� 104

Lmol�1 cm�1).

Cyclic voltammetry experiments were carried out in

the spectroelectrochemical cell equipped with a platinummillielectrode (area 0.196 cm2) and a platinum wire

counter electrode. A silver wire was used as a quasi-

reference electrode. Its potential was checked vs. the

ferricinium/ferrocene couple (Fcþ/Fc) before and after

each experiment. The electrolytic media involved meth-

ylene chloride (CH2Cl2 – Aldrich) and 0.5 M tetrabu-

tylammonium hexafluorophosphate (TBAHP – Fluka)

as supporting electrolyte. All solutions were preparedand transferred into the cell in a glove box containing

dry, oxygen-free (<1 vpm) argon, at room temperature.

Electrochemistry experiments were performed with an

EG&G PAR 273A potentiostat with positive feedback

compensation. Based on repetitive measurements, ab-

solute errors on potentials were found to be around �5

mV.

The spectroelectrochemical cell is sketched in Fig. 1.The body of the cell and all the parts are made of Tef-

lon� in order to be compatible with a broad variety of

solvents. Viton� O-rings are used where needed in order

to ensure gas-tightness. The cell can be coupled indif-

ferently to UV–Vis, IR, Raman or fluorescence spec-

trometers. The working electrode is a 5 mm diameter

disk of polished platinum (or gold, or vitreous carbon)

inserted in a Teflon� rod. It is mounted in the centralwell on a micrometer screw that permits fine adjustment

of the distance between the electrode and the optical

window, while maintaining the surface of the electrode

parallel to the window. A distance of 25–200 lm be-

tween the surface of the electrode and the optical win-

dow was typically used in our experiments. This was

adjusted in order to obtain the best fluorescence and

N

O

O

C5H11 NN

OO

O O

C5H11C5H11

Cl

Cl

Cl

Cl

SO2Cl2,I2, iodobenzène nitrobenzène

yield = 79%

lene dipentylimide (1).

Fig. 2. Optical and fluorescence spectra of 1 (0.5 mM) in 0.5 M

TBAHP/CH2Cl2 (excitation wavelength: kexc ¼ 480 nm).

Fig. 1. Schematic view of the spectroelectrochemical cell. The diameter

of the optical window is 40 mm.

M. Dias et al. / Electrochemistry Communications 6 (2004) 325–330 327

electrochemical responses. The conversion time is less

than 1 s at 20 lm and less than 100 s at 200 lm. For

UV–Vis and fluorescence experiments, the optical win-

dow was in silica. In order to use fluorescence detection,

the lower part of the cell was modified (Fig. 1): the ex-citation beam was sent at an angle of 45� relative to the

surface of the working electrode, whereas the fluores-

cence light was collected through another optical fiber

perpendicular to the quartz window. The sheath of op-

tical fibers should not be fluorescent. It should be noted

that these optical fibers are also convenient for Raman

detection.

Experiments were performed by using a QM-4/QuantaMasterTM fluorometer from PTI� with rapid

monochannel detection and continuous excitation

source. The UV–Vis spectrophotometer was a Lambda

19 NIR model from Perkin–Elmer. UV–Vis spectro-

electrochemistry experiments were carried out on a J&M

Tidas 2 multichannel spectrophotometer, coupled to an

Eco Chemie Autolab model 100 potentiostat for elec-

trochemical monitoring and recording.

3. Results and discussion

Fig. 2 accounts for the absorbance (kmax ¼ 425, 480,

520 nm) and fluorescence (kem ¼ 540 nm at kexc ¼ 480

nm) spectra of 1 in 0.5 M TBAHP/CH2Cl2. All fluo-

rescence measurements were carried out with an exci-tation wavelength of 480 nm. The fluorescence quantum

yield has been recorded relative to perylenetetracarb-

oxylic diimide (Uf ¼ 0:94 – Aldrich) by using the

method of Williams et al. [27] and estimated to

Uf ¼ 0:87. This value was in agreement with severalresults reported in the literature [15–24,28].

The electrochemical characterization of 1 was carried

out by using cyclic voltammetry (CV). As expected [24],

its electrochemical behavior is characterized by two fully

chemically reversible, one-electron reduction processes

at )0.38 V/ECS and )0.58 V/ECS, respectively, in 0.5 M

TBAHP/CH2Cl2 (Fig. 3 – CV). The excellent revers-

ibility of the reduction processes was observed underthin layer condition down to 1 mV s�1 (Fig. 3 – TLCV).

In order to test the efficiency of the spectroelectro-

chemical cell, the electrochemical conversion time of our

cell was estimated by chronoamperometry. In thin layer

condition (<50 lm), applying a negative potential step

(0.0 to )0.8 V/ECS) led to a fast reduction of 1 into 12�

and, thus, to the observation of a rapid decrease of the

current (IUPAC convention). The shape of this decreaseallowed to estimate a conversion time better than 5 s

(Fig. 4).

During this experiment, the vicinity of the working

electrode was irradiated at 480 nm. The fluorescence

intensity, expected at 540 nm, was monitored and re-

corded (Fig. 4, left Y -axis). First of all, it should be

noted that the decrease of the fluorescence and the de-

crease of the current are synchronized. The electrontransfer process induces a normal quenching pathway,

this corresponding to a simple electron transfer process

based on the Rehm–Weller theory [29]. Then, applying

the successive steps (0.0!)0.8! 0.0!)0.8 V/ECS. . .)leads to observe a reversible quenching, independent of

the concentration of 1 and of the nature of the working

electrode. Finally, our setup allows to follow the fluo-

rescence as a function of time with a good signal-to-noise ratio.

The typical ‘‘spectrofluoroelectrogram’’ shown in

Fig. 5 corresponds to a 3D representation of the series of

Fig. 4. Chronoamperometry (0 to )0.8 V/ECS) and in situ fluorescence

observed at 540 nm (kexc ¼ 480 nm) of 1 (ca. 0.5 mM) in 0.5 M

TBAHP/CH2Cl2.

Fig. 5. Fluorescence spectroelectrochemistry of 1 (ca. 0.1 mM) in

TLCV (�50 lm) in 0.5 M TBAHP/CH2Cl2.

Fig. 6. (a) Voltafluorogram measured at 540 nm (kexc ¼ 480 nm) of 1

(ca. 0.1 mM) in 0.5 M TBAHP/CH2Cl2 (b) CV vs. time at 5 mV s�1

(solid line) and time derivative of voltafluorogram vs. time (dotted

line).

Fig. 3. CV (top) and TLCV (bottom) of 1 (ca. 0.5 mM) in 0.5 M

TBAHP/CH2Cl2.

328 M. Dias et al. / Electrochemistry Communications 6 (2004) 325–330

fluorescence spectra recorded during a cyclic voltam-

metric experiment (thin layer conditions, 1 mV s�1). The

red curves, also displayed as a function of time, are the

unfolded voltammogram, recorded simultaneously. The

potential decreases from 0 to )0.8 V and then increasesback to 0 V. It can be seen that the reduction of 1 is

M. Dias et al. / Electrochemistry Communications 6 (2004) 325–330 329

paralleled by the disappearance of fluorescence and thatthe quenching of fluorescence coincides with the first

reduction peak.

This point is more clearly evidenced in Fig. 6: the top

half of the figure is a cut of the 3D spectrofluoroelec-

trogram at its 540 nm emission maximum. The top half

is the first derivative with respect to time of that same

curve, docked with the CV curve (unfolded as a function

of time). Now, considering that the fluorescence inten-sity is proportional to concentration, and that the cur-

rent is the first derivative of the charge flowing through

the system, it is obvious that the peak of the first de-

rivative coincides with the peak of the CV correspond-

ing to the first reduction step of 1. Finally, we

demonstrated that the fluorescence can be followed as a

function of the potential with a good signal-to-noise

ratio.Similar results were observed over a broad range of

concentrations and the quenching of fluorescence of 1

was detected down to 0.005 mM.

In order to compare the efficiency of this technique,

the same experiments were performed with UV–Vis

monitored spectroelectrochemistry. It was impossible to

Table 1

Optical bands of 1, 1� and 12� in 0.5 M TBAHP/CH2Cl2.

Compound kmax (nm)

1 425, 480, 520

1� 755, 775

12� 610, 680

Similar results have been observed on perylene, terrylene, qua-

terrylene dimides in 24.

Fig. 7. UV–Vis spectroelectrochemistry of 1 (ca. 0.1 mM in TLCV

(�50 lm) in 0.5 M TBAHP/CH2Cl2. Note that the UV–Vis absor-

bance at a given potential was determined by comparison with a ref-

erence spectrum recorded at the equilibrium potential.

observe the optical bands in such drastic conditions (lowconcentrations). The optical bands of 1� and 12� (Table

1) could be detected by UV–Vis spectroelectrochemistry

down to 0.05 mM (Fig. 7). Thus, the sensitivity of

spectrofluoroelectrochemistry is greater than techniques

coupled to absorption spectroscopy in the same experi-

mental conditions.

4. Conclusion

Contrary to all spectroelectrochemical studies pub-

lished so far in the literature, the new cell described here

is versatile. It was designed and built and can be used forUV–Vis, IRTF, Raman and, now, fluorescence spec-

troelectrochemical measurements over a broad variety

of detection methods (reflection, absorption, diffusion,

monochannel or multichannel).

The possibility of coupling fluorescence and electro-

chemistry opens up new possibilities to probe interfacial

charge transfer interactions and redox intermediates in

the vicinity or on the modified surface of electrodes.Detailed investigations of the electrochemical, optical

and photochemical properties of some perylene deriva-

tives with and without donor moieties are now under-

way and will be reported in the near future.

Acknowledgements

The authors wish to thank Jean-Pierre Verwaerde,

mechanical technician at LASIR, for his clever advice

and talented craftsmanship in the making of the spec-

troelectrochemical cell.

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