High-Performance Electrofluorochromic Switching Devices Using a

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High performance electrofluorochromic switching devices using a novel arylamine-fluorene redox active fluorophore Giuseppina Anna Corrente, Eduardo Fabiano, Massimo La Deda, Francesca Manni, Giuseppe Gigli, Giuseppe Chidichimo, Agostina-Lina Capodilupo, and Amerigo Beneduci ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01656 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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ACS Applied Materials & Interfaces

High performance electrofluorochromic switching devices using a novel arylamine-fluorene redox active fluorophore

Giuseppina A. Corrente,† Eduardo Fabiano,§,∥ Massimo La Deda, † Francesca Manni,‡, Giuseppe Gigli, ‡ Giuseppe Chidichimo,†Agostina-L. Capodilupo,‡* Amerigo Beneduci,†* †Department

of Chemistry and Chemical Technologies, University of Calabria, Via P. Bucci, Cubo 15D, 87036 Arcavacata di Rende (CS), Italy

§Institute

for Microelectronics and Microsystems (CNR-IMM), Via Monteroni, Campus Unisalento, 73100 Lecce, Italy

∥Centre

for Biomolecular Nanotechnologies @UNILE, Istituto Italiano di Tecnologia (IIT), Via Barsanti, 73010 Arnesano, Lecce, Italy

‡CNR

NANOTEC, Institute of Nanotechnology, c/o Campus Ecotekne, University of Salento, via Monteroni, 73100 Lecce, Italy

⊥Dipartimento

di Ingegneria dell'Innovazione, Università del Salento, via Monteroni, 73100 Lecce, Italy

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Keywords: Electrofluorochromism, light modulation, arylamine-fluorene mixed valence, electroactive fluorophore, switching device

ABSTRACT Fluorescent light modulation by small electric potentials, has gained huge interest in the last few years. This phenomenon, called electrofluorochromism, is of the utmost importance for applications in optoelectronic devices. Huge efforts are being addressed to developing electrofluorochromic systems with improved performances. One of the most critical issue is their low cyclability which hampers their widespread use. It mostly depends on the intrinsic reversibility of the electroactive/fluorophore molecular system and on device architecture. Here we show a novel fluorene-based mixed valence electrofluorochromic system which allows direct electrofluorochromic switching and exhibits incomparable electrochemical reversibility and device cyclability of more than 10000 cycles.

Electrofluorochromism is the reversible modulation of the fluorescence state of a material due to a reversible change of its redox state achieved by the application of a dc voltage.1-5 Fluorescent modulation can lead either to an intensity switch (ON/OFF)6-11 or to a colour switch.3,12-15 Therefore, it can be virtually exploited in a wide range of applications such as in displays, sensors and memory devices and molecular logic gates.16,17 Two strategies have been reported to design electrofluorochromic materials: molecular dyads,17-19 made of a fluorophore linked to a redox unit which acts as quencher through photoinduced electron/energy transfer processes between the excited state of the fluorophore and the redox unit; and redox active fluorophores,2,3,11,13 for which a direct oxidation or reduction of the fluorophore leads to a change of the fluorescence emission. In the latter case, the stability of the radical ion formed as a consequence of the redox process, is crucial for obtaining highly reversible electrofluorochromism. Among the small organic redox active ACS Paragon Plus Environment

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fluorophores, those belonging to the class of mixed valence compounds,20 are extremely interesting due to the stabilization of their radical ions by electron coupling between the redox centers.21,22 The architecture of a mixed valence electrofluorochromic system (MVEFC), is that shown in Scheme 1a. Two (or more) identical redox centers are separated by a central fluorescence unit which plays also the role of electron coupling bridge. Thus, the design of MVEFCs relies on the choice of the redox centers and of the fluorophore bridging unit. Direct electrofluorochromic switching has been early reported for the class of electron acceptor thienoviologen dications (TV2+) which are easily reduced to a radical cation mixed valence state (TV●+).3,13 The essential components of an electrofluorochromic device are the conductive substrates, the supporting electrolyte, the anode and the cathode, of which at least one must be electrofluorochromic. In the example cited before, the electrofluorochromic system TV2+/ TV●+ acts as cathode as well. It is clear that one can design an anode with electrofluorochromic properties that, in principle, can be used in combination with an electrofluorochromic cathode working as complementary electrochemical units. In this case, the light switching properties would be determined by the properties of each electrofluorochromic unit and their interactions, possibly, allowing modulation of the emitted light in a more structured way and thus, extending the range of applications of EFC devices. A simple strategy to obtain MVEFC systems with anodic character is to use arylamine substituents as redox centers.23-24 To our knowledge, only two examples of these systems have been reported in the literature, one incorporating a diphenyl bridge23 and the other one an indole-carbazole bridge.24 Moreover, these systems are very versatile because they can be designed to have more than three redox states (as in the case of diamine compounds) by either introducing three22 or four21 redox centers around the central bridge, as well as by designing starburst-like systems where each N-bridge unit is connected to a central N-atom.23 Therefore, we believe that there is the need to develop such type of very promising electrofluorochromic systems and stable and highly performance EFC devices based on them. In order to develop a system with unique electrofluorochromic properties, easy processability and low cost, here we have designed the anodic MVEFC molecule 9,9-bis(2-(2-methoxyethoxy)ethyl)ACS Paragon Plus Environment


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N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-9H-fluorene-2,7-diamine, in which a highly fluorescent fluorene bridge is functionalized with the 4-methoxy diarylamine redox units on the 2,7-positions and with two methoxyethoxyethyl chains on the 9-position (Scheme 1b). Fluorene amino-derivatives are promising candidates as anodic electroactive materials in electrofluorochromic devices,12 due to their electrochemical, photoluminescence (PL) and photoinduced hole transporting properties.25,26 4-Methoxyarylamines were selected as the redox sites because they form stable N•+ radicals at low potentials and have excellent electrochemical reversibility.21,22 As we will see shortly, the methoxyethoxyethyl substituents serve as compatibilization units toward the components of the polymer matrix used to form the electrofluorochromic film. This compound was prepared with a straightforward synthetic pathway carrying out the synthesis under microwave irradiation to obtain the desired products in a short time and with high yields. The detailed synthetic routes and its characterizations are depicted in Supporting Information.

Scheme 1. a) generalized mixed valence electrofluorochromic molecular system. b) MVEFC 9,9-bis(2-(2-methoxyethoxy)ethyl)-N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-9H-fluorene-2,7diamine, where the functional units are highlighted.


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In CH2Cl2 solution the MV fluorene derivative (Scheme 1b) exhibits a structured absorption spectrum in the UV range up to about 400 nm (Figure 1a), with a strong absorption band peaked at 375 nm and other two low wavelength bands at 310 nm and 240 nm. Time-dependent density functional theory (TD-DFT) calculations show that the peak at 375 nm is a -* transition, whereas the peak at about 310 nm is originated by two degenerate excitations both having N-to-bridge-CT (NBCT) character; finally, the peak at 240 nm can be attributed to a transition having a mixed -*/ NBCT character (Figure S1 and Table S1). By UV illumination, a peak fluorescence at 426 nm has been recorded, due to a radiative deactivation from the -* excited-state, (Figure 1a) with a fluorescence quantum yield as high as 49%.

Figure 1. Properties of the MV fluorene derivative. (a) Absorption and emission spectra of the neutral state (CH2Cl2); (b) cyclic voltammetry in CH2Cl2/TBAPF6 (0.1 M) at 100 mV/s; (c) cyclic ACS Paragon Plus Environment


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voltammetry in NMP/TBAPF6 (0.1 M) at 50 mV/s under repeated scans. (d) Spectroelectrochemistry of the MV fluorine derivative. The potentials in the inset are vs. AgCl/Ag.

Cyclic voltammetry (CV) measurements of the MV fluorene derivative (Figure 1b and c), show two well-defined one-electron reversible oxidations, at half wave potentials of E1/2(1) = +0.55 and E1/2(2) +0.85 V vs. AgCl/Ag (Figure 1b), which correspond to the formation of the radical cation (𝑁 ∙ + ) and the dication (𝑁 + + ) species, respectively. The calculated electrochemical splitting between the two redox waves is rather high either in CH2Cl2/TBAPF6 (low dielectric medium with a high tendency to ion pairing) or in NMP/TBAPF6 (high dielectric medium, with high solvation and charge shielding effects) (Table S2), indicating a strong contribution of the resonance coupling energy to the Gibbs free energy of comproportionation.21-27 A strong evidence of this electron coupling is the intense and wide near infrared band in the absorption spectrum of the radical cation (Figure 1d), due to an optical induced charge transfer transition (IVCT), typical of arylamine MVs of Robin and Day class II and III.28 This excitation is also nicely predicted by TD-DFT that locates an IVCT excitation at 1453 nm. A second less bright excitation is found at about 558 nm and displays a mixed IVCT/NBCT character (Table S1). Extensive cyclization (100 cycles) around the first redox cycle as well as across both the redox waves (Figure 1c and Figure S2), shows the invariance of the 𝑁0/𝑁

∙+

and 𝑁0/𝑁

++

cycles to

repeated scans, highlighting the high reversibility of the electroactive fluorophore and the stability of the radical cations against side reactions. The radical cation species can be also produced by chemical oxidation of the MV fluorene derivative by reaction with SbCl5 in CH2Cl2 solution (Figure 2a). If the emission spectrum is quantitatively monitored as function of the amount of oxidant added, a progressive quenching of the fluorescence can be observed, up to its complete disappearing when the starting neutral species is quantitatively converted to the monocation one. Hence, the radical cation species is not emissive.


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This behavior can be modeled considering that the absorption spectrum of the oxidized species presents the lowest-energy band positioned at about 1480 nm. Oxidation involves the formation of an electronic hole in the HOMO; following irradiation, an electronic promotion from a low-energy full orbital (HOMO-1) towards this hole takes place, responsible for the band at 1480 nm. When the system relaxes, the return of the electron to the starting point does not involve the emission of a photon, since the difference in energy between HOMO and HOMO-1 is too low to favor a radiative deactivation, and a non-radiative path become paramount. This result, combined to the electrochemical study described before, points out that the MV fluorene derivative can be used as an intrinsically switchable fluorophore in electrofluorochromic devices.

Figure 2. (a) Fluorescence quenching of the MVEFC (4.0 × 10−5 M in CH2Cl2) upon SbCl5 titration (Fluorescence contrast ratio = Ineutral/ Imonocation = 746); (b) Fluorescence spectra as a function of the voltage bias applied to the ITO/EFC/ITO device. Each spectrum has been acquired after 60 s of application of the bias. The EFC layer contains 5% (w/w) of the electroactive fluorophore. Film thickness = 5 μm.

In order to verify it we have made ITO/EFC film/ITO devices where all-in-one single electrofluorochromic layers were used. These layers were prepared according to a method developed previously by our group.4,22,29 Briefly, the initial mixture containing a thermoplastic polymer ACS Paragon Plus Environment


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(Polyvinylformale, PVF), a solvent plasticizer (N-Methylpyrrolidinone, NMP), the supporting electrolyte (TBAPF6), the MV fluorene anodic component and the ethylviologen perchlorate (EV(ClO4)2) chosen as cathode, was laminated between two ITO conductive slabs at 100 °C. The EV++ is an ideal cathode because, firstly, it has a first reduction potential at about -0.55 V vs. AgCl/Ag30 that closely matched the first oxidation potential of the anodic MV fluorene and, secondly, the EV++/ EV●+ is a highly reversible redox couple.30 The MV fluorene derivative is very soluble in NMP due to the polar methoxyethoxyethyl chains (Scheme 1b) and can be easily incorporated into the PVF matrix, therefore avoiding possible de-mixing during the working operation of the device and contributing to its long term stability. The fluorescence spectrum of the device containing 5% (w/w) of the MV fluorene derivative, in the OFF state, is shown in Figure 2b. The emission band is slightly larger than that acquired in CH2Cl2 solution and exhibits a shoulder at about 450 nm close to the emission maximum. This has been also observed in NMP solution and is due to the increase of the medium polarity (Figure S3). The bluewhite light emitted (excitation at 370 nm) is very intense (inset of Figure 2b), due to the high fluorescence quantum yield in the device (63%).31 When a dc voltage bias is applied to the EFC cell, an electrofluorochromic quenching effect occurs that increases with the bias applied (Figure 2b). The analysis of the spectral sequence of Figure 2b and Figure S4 (relative to the devices with different composition), reveals that there is an onset potential, common to all the devices studied, above which the electrofluorochromic quenching begins. This can be clearly seen in Figure 3 that reports the fluorescence loss as a function of the bias applied and the amount of MV fluorene in the EFC film.


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Figure 3. a) EFC effect measured in terms on fluorescence loss as a function of the potential bias applied to the ITO/EFC/ITO device and as a function of the amount of electroactive fluorophore MV fluorine incorporated into the polymer film. b) Electrochemical processes occurring in the device during the application of different switching voltage biases at the ITO electrodes and in the bulk of the electrofluorochromic polymer layer.

For all the devices, the fluorescence loss follows a sigmoidal trend. No loss is observed up to about 0.75 V, above which it undergoes a stepwise increase within about 1.50 V. In this potential range, more than 90% of the photoluminescence quenching occurs (Figure 3). Then, the fluorescence loss tends to reach a plateau value close to 100%. Inspection of the CV (Figure S5a) and of the spectroelectrochemistry of the device (Figure S5b), allows us to deduce that, just above the onset potential, a first oxidation process occurs with the concomitant rising up of the IVCT band of the MV fluorophore (Figure S5b). Thus, this potential value corresponds to the initial oxidation of the fluorophore (Figure 3b). On the other hand, reduction



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of the EV dication occurs almost at the same time (Figure 3b) on the ITO substrate opposite to that where the fluorophore oxidation occurs), as clearly evidenced by the appearance of the typical absorption bands of the viologen radical cation (EV●+) between 550 and 750 nm (Figure S5b) and by the fact that the first redox process in the CV of the device is a superposition of the two redox waves associated to the N/N●+ and EV++/EV●+ couples. Production of the radical cation of the electroactive fluorophore is almost complete at about 1.31.5 V (Figure S4a), thus causing most of the electrofluorochromic quenching effect. Fluorescence contrast ratios (IOFF/ION) of up to 40 can be obtained after charging the device for 60 s above 1.5 V. Above this voltage bias, the monocation radical MVEFC●+ undergoes further oxidation to the dication species (MVEFC++) which can react with the neutral fluorophore (MVEFC) present in the bulk of the electrofluorochromic layer, according to the favored comproportionation reaction (Table S2) and, eventually, with the highly reducing neutral viologen, to form again the monocation radical and thus leading to a further photoluminescence quenching (Figure 3b). The electrofluorochromic performances of the devices were further evaluated by switching them between alternating ON and OFF pulses with different duration and intensity (Figure 4 and Table S3 and S4). Figure 4 depicts the easy of light intensity modulation by changing the square wave potential and highlights the highly reversible switching behavior. The contrast ratio and the switching times, measured for several switching conditions and for the different devices studied, are reported in Table 1. It is worth to noticing here that a comparison among the various EFC systems already reported can be hardly done on a quantitative basis due to the lack of standardized procedures for comparing the EFC performances and by the lack of experimental data related to devices, especially on switching responses and cyclability. Nonetheless, Table 1 shows that relatively high contrast ratios (up to 30) can be achieved at switching times in the sub-second timescale. In general, the response times to achieve such contrasts, even for systems based on thin EFC polymer films, are longer.1-6,10,13 Video S1 shows the electrofluorochromic modulation of the light intensity emitted by the device with 5%


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of the fluorophore, upon the application of a 3 V bias for 2 s (ON) and an inverse bias of -0.5 V for 2 s (OFF). Finally, the long term stability of the EFC device was also studied. Figure 4b displays the loss of contrast ratio as a function of the number of EFC cycles. The device experiences a contrast loss smaller than 10 % in the first 4000 cycles and is very stable up to about 10000 cycles at which the loss of contrast ratio is about 20-25%. The stability significantly deteriorates for longer cycles, with a loss of contrast close to 50% above 15000 cycles. These high cyclability values have never been reported, to the best of our knowledge. Indeed, from an accurate inspection of the literature, long term cyclability studies are almost absent and the papers in which they are reported or from which cyclability data can be estimated, show significant contrast losses (> 5%) just after a few tens1-6,10 or a few hundreds of cycles.13 We believe that the remarkable stability of our device stems on the intrinsic electrochemical stability of the electroactive components, as well as on the fact that these are incorporated into a single layer phase formed by a polymeric gel (polymer + solvent plasticizer + electrolyte) where they are homogeneously dissolved. The working principle of this single-layer device is based on the migration of the components with different oxidation states and charge toward the electrodes and from the electrodes to the bulk of the film, through the polymeric viscous medium (Figure 3b).13,29 In conclusion, here we have shown that the MV fluorene derivative is a very efficient EFC anodic system due to the high stability of its radical cation which is not emissive. Incorporation of this anode into polymeric EFC films then leads to devices with very fast response speed corresponding to relatively high fluorescence on/off contrast, and long-term cyclic stability, all distinctive properties for a real-life application of EFC devices.



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Figure 4. (a) ON/OFF light switching at 430 nm of the device containing 5% of the electroactive fluorophore. Different consecutive potential pulse sequences are shown, generating increasing contrast ratios. (b) Long term stability of the device measured in terms of loss of contrast ratio as a function of the number of electrofluorochromic switching cycles.

Table 1. Electrofluorochromic switching performances of the MV fluorene based devices


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Pulse sequence % (w/w)

V1_V2(V)-t1_t2 (s)

IOFF/ION (𝝈)

𝝉𝟏 (𝝈) (s)

𝝉𝟐 (𝝈) (s)

0_1 - 5_10

2.7 (0.1)

0.6 (0.1)

0.47 (0.01)

0_1.6 - 5_10

8.6 (0.2)

0.4 (0.2)

0.2 (0.1)

0_6 - 20_1

28.1 (0.9)

0.3 (0.1)

9 (0.2)

-0.5_1 - 2_2

2.2 (0.1)

1.03 (0.06)

0.7 (0.3)

-0.5_1.5 - 4_2

7 (0.1)

0.67 (0.04)

0.9 (0.2)

-0.5_3 - 4_1

21 (0.1)

0.95 (0.01)

1.9 (0.1)

-0.5_1.5 - 5_2

4.2 (0.1)

1.1 (0.1)

2.1 (0.1)

-0.5_1.75 - 6.5_2

5.1 (0.1)

0.9 (0.1)

1.8 (0.1)

-0.5_2 - 7_2

7.5 (0.3)

0.6 (0.3)

1.9 (0.2)

MV fluorene

10

5

1

ASSOCIATED CONTENT Supporting Information. Supporting Information. TD-DFT data (Figures S1 and Table S1), electrochemical properties in different media (Table S2 and Figure S2), absorption and emission data in NMP solution (Figure S3), emission spectroelectrochemistry of the EFC devices with different film composition (Figure S4), cyclic voltammetry and absorption spectroelectrochemistry of the device (Figure S5), electrofluorochromic performances of the devices at different switching potentials (Table S3 and S4). Detailed experimental section, synthesis (Scheme S1) and NMR spectra (Figures S6-S9).

AUTHOR INFORMATION Corresponding Author * [email protected] (A.B.) * [email protected] (A.-L. C.)

ORCID Amerigo Beneduci: 0000-0003-1185-9470 Agostina-Lina Capodilupo: 0000-0003-4755-4011

Notes ACS Paragon Plus Environment


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The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful to the Ministero dell’Istruzione dell’Universita e della Ricerca Italiano (MIUR) and the University of Calabria for supporting this project in the framework of the ex 60% budget grant. In addition, the authors gratefully acknowledge the project PERSEO-“PERrovskite-based Solar cells: towards high Efficiency and lOng-term stability” (Bando PRIN 2015-Italian Ministry of University and Scientific Research (MIUR) Decreto Direttoriale 4 novembre 2015 n. 2488, project number 20155LECAJ) and the FISR-C.N.R research project “TECNOMED - Research Centre for the application of Nanotechnology and Photonics to Precision Medicine”-CUP B83B17000010001 for funding.

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(30) Rosseinsky, D.; Monk, P.; Mortimer, R., Conjugated Conducting Polymers. In Electrochromism and Electrochromic Devices, Eds.; Cambridge University Press: Cambridge, 2007, 312−340. (31) This value is higher than those obtained in CH2Cl2 solution and in NMP solution (36.5 %), due to the restricted intramolecular motion (RIM) of the fluorophore in the more viscous polymer gel medium. The aggregation induced emission (AIE) has been ruled out because the emission quantum yield (EQY) did not show any increase with the increase of the fluorophore concentration in solution. Moreover, an increase of the radiative kinetic constant with the dielectric constant of the environment (from dichloromethane solution to N-methyl pyrrolidone/PVF gel), has been ruled out because the EQY in NMP is lower than that in CH2Cl2.


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High-Performance Electrofluorochromic Switching Devices Using a

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