Highly Stable Perylenediimide-Based Self-Assembled Monolayers Studied with Spectroelectrochemistry
Sihame Bkhach, Yann Le Duc, Olivier AlvÞque, Christelle Gautier, Pitrick Hudhomme,* and Eric Levillain*
[a]Perylenediimide (PDI)-based self-assembled monolayers (SAMs) have been studied by using quartz crystal microbalance, X-ray photoelectron spectroscopy, cyclic voltammetry, and spectro- electrochemistry (SEC). The high stability of PDI-based SAMs has allowed very low signals to be probed by using absorption and emission SEC and by extracting voltabsorptograms. The wavelengths are reported for the absorption maxima of the PDI, anion radical, and dianion species. In contrast, the magni- tudes of the molar extinction coefficient of the reduced forms were not preserved in the SAM. The quenching of PDI fluores- cence was confirmed on a gold substrate.
Over the last decades, the modification of surfaces with the formation of self-assembled monolayers (SAMs) has constantly been under extensive investigation.[1]The ease of SAM prepa- ration, their stability, and the possibility to introduce different functional groups makes it easy to obtain surfaces that exhibit tailored properties.[2–4]
Our interest in using these modified surfaces concerns the investigation and identification of electroactive species at dif- ferent redox states or products arising from redox reactions by using time-resolved absorption and emission spectroelectro- chemistry. Contrary to molecules studied in solution, this ap- proach is still uncommon for these materials,[5–10]because the nanometric scale of the electroactive layer remains a major lim- iting factor to characterizing SAMs with the usual absorption and fluorescence spectroscopic methods. Thereby, such charac- terizations require the development of efficient time-resolved spectroelectrochemical measurements that are capable of ac- curately monitoring the evolution of the spectroscopic signa- ture as a function of an electrical perturbation, such as a poten- tial step or linear scan, and of probing very low-intensity sig- nals at high signal-to-noise ratios.[11]
We were interested to work with redox SAMs containing a perylenediimide (PDI) core, which exhibit an almost quantita- tive fluorescence quantum yield and high photochemical sta-
bility.[12, 13] Moreover, this system presents a fully electron n- type reversible electrochemical process, and has been used for the first time-resolved emission spectroelectrochemistry per- formed in solution.[14, 15]PDI is one of the most important chro- mophore/fluorophore materials for various applications, be- cause of challenges associated with assembling these mole- cules on technologically important surfaces for potential practi- cal utility.[16–18]
In this Communication, we report the first absorption and fluorescence spectroelectrochemical study on PDI-based SAMs.
These experiments were carried out by using quartz crystal mi- crobalance (QCM), X-ray photoelectron spectroscopy (XPS), and spectroelectrochemistry (SEC) techniques.
We opted for a dialkyl disulfide PDI rather than an alkane- thiol PDI derivative, because the electrochemical stability of SAMs prepared from the latter functionality have been shown to be weaker.[19] PDI derivative 4 was synthesized in three steps (Scheme 1 as well as Scheme S1 in the Supporting Infor-
mation). The first one involves the condensation of starting material1 with 3-aminopentane and 12-aminododecan-1-ol in a stoichiometric ratio.[20] Asymmetrical PDI 2 was separated from the symmetrical compounds by using silica gel column chromatography. The thioester functionality was introduced by using a Mitsunobu reaction, affording PDI3.[21]The thioacetate group was finally deprotected by using CsOH,[22]and PDI disul- fide4was isolated after purification with silica gel chromatog- raphy in an overall yield of 17 %, starting from1.
SAMs were prepared by dipping 0.2 cm2 gold substrates in a 0.5 mmsolution of PDI4(i.e. 1 mmPDI moieties) in CH2Cl2at 208C. The gold substrates were either gold electrodes ob- tained through the physical vapor deposition of approximately 10 nm of chromium and 100 nm of gold onto a glass substrate, or commercial AT-cut 9 MHz gold-coated quartz crystal oscilla- tors. QCM is the ultimate tool for following the formation of [a]S. Bkhach, Dr. Y. Le Duc, Dr. O. AlvÞque, Dr. C. Gautier, Dr. P. Hudhomme,
Dr. E. Levillain
Laboratoire MOLTECH-Anjou Universit d’Angers/CNRS UMR 6200
2 Boulevard Lavoisier, 49045 Angers Cedex (France) Fax: (+33) (0)2 41 73 54 05
E-mail: [email protected] [email protected]
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/celc.201600034.
Scheme 1.Synthesis of PDI derivative4: a) 3-aminopentane, H2N(CH2)12OH, reflux toluene, 72 h; b) DEAD, PPh3, CH3COSH, CH2Cl208C; c) CsOH.H2O, THF- MeOH, 08C.
monolayers, because it allows the in situ display of the mass variation induced by the chemisorption reaction on an Au- coated quartz crystal. Immersed in a 20 mL CH2Cl2solution, an AT-cut 9 MHz gold-coated quartz crystal oscillator was stabi- lized within 1 min. The injection of 10mmol of disulfide4(final concentration of 0.5 mm) involves a drastic frequency decrease, corresponding to the adsorption of 33.60.2 ng of PDI4(i.e.
1 Hz=1 ng), giving rise to a surface coverage of 2.30
0.01.10 10mol cm 2 after 15 min (Figure 1). As the frequency variation remained stable after 15 min, this immersion time was used for all electrode modifications.
To estimate the elemental composition, XPS measurements were recorded on a SAM formed after 15 min immersion of a gold electrode in a 0.5 mmsolution of 4 in CH2Cl2. Integra- tion of the O 1s, N 1s, C 1s, Cl 2p, S 2p, and Au 4f signals gave access to the atomic composition of the modified gold elec- trode (Table 1).
The atomic percentage of each element constituting the SAM, determined from the core-level spectrum, was in agree- ment with the expected composition. Indeed, the Cl/O, Cl/N, and Cl/S ratios were close to 1, 2, and 4, respectively, as ex- pected, with regard to the atomic formula of 4 (C82H78Cl8N4O8S2).
To probe the electrochemical behaviour, cyclic voltammetry and subsequent absorption SEC experiments (A-SEC)[11] were performed on a SAM of 4in 0.1mTBAPF6/CH2Cl2, which were then compared to4in solution. As expected,[15]the cyclic vol- tammograms (CVs) of 4 are characterized by two fully chemi- cally reversible one-electron reduction processes at 0.62 and 0.82 V in solution and at 0.71 and 0.84 V in the SAM (vs.
AgNO3/Ag) (Figure 2).The redox stability of SAMs of4was es- tablished by using A-SEC after two electrochemical cycles (Figure 3), and the absorption bands characteristic of 4 . and 42 were assigned from the derivative of the cyclic voltabsorp- togram [DCVA=d(absorbance)/dt] from the SAM of4(Table 2), and then compared to those of4 that were observed in solu- tion under thin-layer conditions (Figure 4).
As expected,[6]the wavelengths of the absorption maxima of the4,4 ., and42 species in SAMs are much the same as in
Figure 1.Frequency change (DF) versus time (black dots) during disulfide4 injection in CH2Cl2on an AT-cut 9 MHz gold-coated quartz crystal oscillator.
Fit (red solid line) of the experimental data with Langmuir adsorption iso- therm (kobs=4.350.01 10-2m1s 1andDFmax=33.6+/ 0.2 Hz), according to Ref. [23].
Table 1.Atomic composition of SAMs from XPS data.
Amount [at %]
O N C Cl Au S
4.0 1.8 50.3 3.8 39.1 1.0
Figure 2.Cyclic voltammetry in 0.1mBu4NPF6/CH2Cl2at 100 mV s 1and 293 K:A) 0.1mof4on a gold electrode during three redox cycles; B) SAM of4over four redox cycles. The scan-rate dependence of the current peaks confirmed a surface-confined redox species and the surface coverage (i.e.
1.920.08.10 10mol cm2), deduced by the integration of the voltammetric peak, was estimated from two generalized Gaussian function of equal area (red and blue lines) on the basis of the procedure from Ref. [24].
solution. It is noteworthy that the magnitude of the molar ex- tinction coefficient of4is preserved in a SAM, which is not the case for the reduced forms4 .and42 (Table 2).
To take advantage of the fluorescence emission of PDI deriv- ative 4, we attempted fluorescence SEC experiments (F-SEC)[11]
on the corresponding SAMs. Although the classical energy- transfer theory suggests an explicit surface damping mecha- nism of luminescence in the range of 1–100 ,[25] several re- ports have indicated a non-quenching of fluorescence in SAMs on gold.[3, 26–30]In this context, despite several attempts in dras- tic conditions (i.e. through rinsing baths and CV checks, etc.), we were unable to detect any luminescence signal from the SAMs of 4 or from the SAMs of 4 diluted with alkanethiols.
Similar results were obtained for the SAMs deposited on indium tin oxide (ITO) substrates.
To ensure that our CCD camera was able to detect a very low intensity of luminescence, we performed F-SEC of4in so-
lution under thin-layer conditions at an analytical concentra- tion equivalent to the surface coverage of the SAM of4 (i.e. 6 10 11mol, corresponding to a thin layer close to 60mm, a 0.2 cm2electrode area, and a 5 10 5manalytical concentra- tion). In such dilute conditions, the fluorescence of4could be measured and even monitored during a cyclic voltammetric experiment (Figure 5), confirming the full disappearance of fluorescence for the anion radical (4 .) and dianion (42 ) species.[15]
A possible way to prove the fluorescent quenching in the SAM involves desorbing the immobilized molecules. We chose to add alkanethiols (non-fluorescent molecules) to the solution near the Au–PDI substrate in order to induce the desorption of 4 through exchange reactions.[1, 31–33] Very quickly, we could again observe the characteristic intense fluorescence of the PDI unit (Figure 6).[8]
Accordingly, the molecular luminescence of 4 is quenched by the metallic substrate, validating the classical energy-trans- fer theory.[25]
To conclude, this work reports pioneering absorption and lu- minescence spectroelectrochemical studies on PDI-based SAMs and shows that the electrochemical behavior of PDI in solution is preserved on an Au substrate. Further works will aim to es- tablish structure/reactivity relationships of mixed PDI/alkane- thiol-based SAMs to probe, amongst others, the impact of sur- face coverage on the magnitude of the molar extinction coeffi- cients of the reduced forms [i.e. anion radical (4 .) and dianion (42 ) species].
Figure 3. A-SEC of the SAM from PDI4in 0.1mBu4NPF6/CH2Cl2at 10 mV s1and 293 K over two redox cycles. A) 3D representation:xaxis=wavelength,yax- is=time, andzaxis=absorbance. B) Comparison betweentvs.iandtvs. d(absorbance)/dtat 755 nm, characteristic of anion radical4 .. Note that the ab- sorbance at a given potential was determined by comparison with a reference spectrum recorded at the equilibrium potential.
Table 2.Absorption bands (standard deviation=2 nm) and molar extinc- tion coefficients (standard deviation=25 %) in 0.1mBu4NPF6/CH2Cl2.
Species In solution In SAM
4 520 nm
(50 000m1cm 1)[a]
540 nm
(54 000m 1cm 1)[b]
4 . 760 nm
(115 000m 1cm1)[a]
755 nm
(15 000m 1cm 1)[b]
42 680 nm
(110 000m 1cm1)[a]
670 nm
(27 000m 1cm 1)[b]
[a] FromA=e‘C(Beer’s law, whereA=absorbance,e=molar absorptivi- ty, ‘=cell path length, andC=concentration) with‘=60mm. [b] From A=1000e G(whereG=coverage) withG=1.92.10 10mol cm2.
Experimental Section
Electrochemistry and time-resolved spectroelectrochemistry in so- lution were performed by using an already-described homemade cell.[4, 6] Electrochemical measurements were carried out by using a platinum wire counter electrode and a silver wire quasi-reference electrode, with a Biologic SP-150 potentiostat driven by EC-Lab
software including ohmic drop compensation. Experiments were recorded in dry HPLC-grade acetonitrile and dichloromethane with tetrabutylammonium hexafluorophosphate (Bu4NPF6, electrochemi- cal grade, Fluka) as the supporting electrolyte. All solutions were prepared and transferred into the spectroelectrochemical cell in a glove box containing dry, oxygen-free (<1 ppm) argon at room temperature.
Figure 4. A-SEC of 5 104mof4in 0.1mBu4NPF6/CH2Cl2at 10 mV s 1and 293 K over two redox cycles under thin-layer conditions (ca. 60mm). A) 3D repre- sentation :xaxis=wavelength,yaxis=time, andzaxis=absorbance. B) Comparison betweentvs.iandtvs. d(absorbance)/dtat 760 nm, characteristic of anion radical4 .. Note that the absorbance at a given potential was determined by comparison with a reference spectrum recorded at the equilibrium potential.
Figure 5.F-SEC of 5 105m of 4in 0.1mBu4NPF6/CH2Cl2at 10 mV s1and 293 K over two redox cycles, in a thin layer close to 60mm, corresponding to a SAM with a surface coverage close to 3.0 1010mol cm 2. Excitation at 495 nm and detection by reflection on an Au electrode between 380 and 980 nm.
A) 3D graph:xaxis=wavelength,yaxis=time, andzaxis=fluorescence intensity. B) Comparison betweentvs.iandtvs. d(fluorescence)/dtat 760 nm, char- acteristic of fluorescence emission of4.
Spectrophotometric measurements were carried out in direct re- flexing mode on the working electrode (i.e. Pt or glassy carbon) with a homemade bench composed of different Princeton Instru- ments modules (light sources, fibers, monochromators, spectrosco- py camera, and software). The connection between the light source, the cell, and the spectrophotometer is ensured through a “Y-shaped” optical fiber bundle: 18 fibers guide the light to the cell and 19 fibers collect the reflected light from the cell to visible (320–1080 nm/maximum acquisition frequency 2 MHz) and IR (900–1700 nm/maximum acquisition frequency 8 MHz) CCD detec- tors. The sensitivity of the spectroscopic measurement [<3 elec- trons at 100 kHz and <13 electrons at 2 MHz between 320 and 1080 nm; 400 electrons (high gain) and 5000 electrons (low gain) between 900 nm and 1700 nm] allows a spectroelectrochemistry experiment to be performed under the usual conditions of electro- chemistry.
Acknowledgements
The authors gratefully acknowledge the MENRT and Angers Loire Metropole for Ph.D. and post-doctoral grants to S.B. and Y.L.D., respectively.
Keywords: fluorescence · perylenediimide · photoelectron spectroscopy · self-assembled monolayers · spectroelectrochemistry
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Manuscript received : January 22, 2016 Accepted Article published: March 2, 2016 Final Article published:&& &&, 2016 Figure 6.Desorption of SAM from4in 0.1mBu4NPF6/CH2Cl2by injection of
alkanethiols att=10 s. 3D representation:xaxis=wavelength,yaxis=time, andzaxis=fluorescence intensity. Excitation at 495 nm and detection by re- flection on an Au electrode between 380 nm and 980 nm.T=293 K.
S. Bkhach, Y. Le Duc, O. AlvÞque, C. Gautier, P. Hudhomme,* E. Levillain*
&&–&&
Highly Stable Perylenediimide-Based Self-Assembled Monolayers Studied with Spectroelectrochemistry
Tailor made:The first absorption and fluorescence spectroelectrochemical studies on perylenediimide-based self- assembled monolayers are reported.
These experiments are carried out by using quartz crystal microbalance, X-ray photoelectron spectroscopy, cyclic vol- tammetry, and spectroelectrochemistry.
