Thiazolothiazole Fluorophores Exhibiting Strong ... - ACS Publications

May 8, 2017 - Department of Chemistry, University of North Carolina at Charlotte, Charlotte, North Carolina 28223, United States. •S Supporting Info...
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Thiazolothiazole Fluorophores Exhibiting Strong Fluorescence and Viologen-Like Reversible Electrochromism Alexis N. Woodward, Justin M. Kolesar, Sara R. Hall, Nemah-Allah Saleh, Daniel S. Jones, and Michael G. Walter* Department of Chemistry, University of North Carolina at Charlotte, Charlotte, North Carolina 28223, United States S Supporting Information *

ABSTRACT: The synthesis, electrochemical, and photophysical characterization of N,N′-dialkylated and N,N′-dibenzylated dipyridinium thiazolo[5,4-d]thiazole derivatives are reported. The thiazolothiazole viologens exhibit strong blue fluorescence with high quantum yields between 0.8−0.96. The dioctyl, dimethyl, and dibenzyl derivatives also show distinctive and reversible yellow to dark blue electrochromism at low reduction potentials. The fused bicyclic thiazolo[5,4-d]thiazole heterocycle allows the alkylated pyridinium groups to remain planar, strongly affecting their electrochemical properties. The singlet quantum yield is greatly enhanced with quaternarization of the peripheral 4-pyridyl groups (ΦF increases from 0.22 to 0.96) while long-lived fluorescence lifetimes were observed between 1.8−2.4 ns. The thiazolothiazole viologens have been characterized using cyclic voltammetry, UV−visible absorbance and fluorescence spectroscopy, spectroelectrochemistry, and time-resolved photoluminescence. The electrochromic properties observed in solution, in addition to their strong fluorescent emission properties, which can be suppressed upon 2 e− reduction, make these materials attractive for multifunctional optoelectronic, electron transfer sensing, and other photochemical applications.

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INTRODUCTION Methyl viologen (N,N′-dimethyl-4,4′-bipyridinium dication MV2+) has a long history of research and development as an herbicide and as a redox couple used in a wide variety of electrochemical and photoelectrochemical processes.1−4 Methyl viologen and its related derivatives undergo reversible oneelectron reductions at low potentials to form very stable radical cations that accompany strong electrochromism, changing clear solutions of MV2+ to dark blue MV+.5,6 Recent applications integrating MV2+ for electrochromic displays and windows have been successfully demonstrated.7−10 New viologen-based structures with highly reducing extended viologen derivatives,11 helical viologens,12 and phosphaviologens,13 have been reported that exhibit unique electrochemical and electrochromic properties. In addition to their electrochromic behavior, their convenient use as a simple organic electron acceptor material has been applied to a diverse range applications such as a reversible redox couple for semiconductor photoelectrodes,14,15 as an electron relay for solarto-hydrogen and solar energy conversion,2−4,16 and for studies © 2017 American Chemical Society

into the dynamics of photoinduced electron transfer from quantum dots to viologen-based acceptors.17−24 Viologenbased materials have also become increasingly utilized in molecular redox-flow battery designs, which enable high performance devices while using inexpensive organic electrolytes and environmental benign solvents.25−29 There continues to be a growing interest in using MV2+ for a variety of electrochemical, photochemical, and biomaterialsrelated applications.11,13 including recent developments using fluorescent sensors to track photoinduced electron transfer.30 It is likely that viologen-based structures with unique multifunctional spectroscopic properties will become important for this area of research. We have synthesized a class of viologen structures (Scheme 1) that demonstrate both reversible electrochromic behavior and high fluorescence quantum efficiency that is deactivated with electrochemical reduction. N,N′-dialkylated and N,N′-dibenzylated 2,5-bis(4-dipyridyl)Received: January 31, 2017 Published: May 8, 2017 8467

DOI: 10.1021/jacs.7b01005 J. Am. Chem. Soc. 2017, 139, 8467−8473

Article

Journal of the American Chemical Society

vis spectra were acquired. Steady-state and time-resolved measurements were taken on a Jobin Yvon-Spex Fluorolog equipped with a 389 nm diode laser for time-resolved photoluminescence (PL) decay measurements (aqueous and ACN solutions 1 μM). PL(t) decay data was fit to single exponential decays using Igor Pro 6.3 software. Quantum yield measurements were conducted using 9,10-diphenylanthracene as a reference with a quantum yield (ΦF) of 0.9 in cyclohexane. Density functional theory calculations (DFT) were performed using Spartan computational software using B3LYP density functional and 6-31G* basis set. Electrochemical (cyclic voltammetry) measurements were performed with a Gamry Reference 600 potentiostat using a Pt disk working electrode, a Pt foil counter electrode, and a saturated calomel (SCE) reference electrode. ACN was found to be poor solvent for dissolving the compounds, therefore cyclic voltammetry (CV) measurements were performed in deaerated DMSO solutions with tetrabutylammonium hexafluorophosphate (TBAH) (0.1 M) supporting electrolyte. Square wave voltammetry was also performed in DMSO using a pulse size of 50 mV and a frequency of 25 Hz. PF6− derivatives of each TTz2+ material were used for all electrochemical characterizations and were obtained by treating aqueous solutions of the TTz2+ compounds with a saturated solution of NH4PF6.11 Crystals of Bz2TTz2+ for X-ray diffraction were obtained through overnight evaporation of aqueous Bz2TTz2+ solutions. A single crystal suitable for analysis was mounted on an Agilent Gemini Ultra diffractometer and kept at 100(1) K during data collection. The structure was solved with the SHElXS structure solution program using direct methods and refined with the SHELX OLEX2 refinement packages.41,42 Preparation of N,N′-Dialkyl or N,N′-Dibenzyl 2,5-bis(4pyridinium) Thiazolo[5,4-d]thiazole Derivatives. The Py2TTz starting material was obtained using previously reported synthetic methods.33,43,44 The dimethyl 2,5-bis(pyridinium) thiazolo[5,4-d]thiazole (Me2TTz2+(BF4−)2) has been reported, however no structural, electrochemical, or spectroscopic characterization is available.45 Trialkylation of Py2TTz (alkylation of the thiazole rings) was avoided by adjusting the temperature and duration of the reaction. In addition, CH3OH Soxhlet extraction during workup helped to remove unreacted Py2TTz and monoalkylated (TTz1+). 2,5-Bis(4-pyridyl)thiazolo[5,4-d]thiazole (Py2TTz). 4-Pyridinecarboxaldehyde (0.44 g, 5.3 mmol) and dithiooxamide (0.20 g, 1.66 mmol) was refluxed in 10 mL anhydrous DMF for 6 h under aerobic conditions. Reaction mixture was allowed to sit overnight, and the precipitated product was collected by filtration and rinsed with water. A yellow solid was collected (0.36 g, 45% yield) 1H NMR and MS data matched previously reported Py2TTz.43 UV−vis λmax (pyridine, ε = M−1cm−1): 357 nm (ε = 32 000). N,N′-Dibenzyl 2,5-Bis(4-pyridinium)thiazolo[5,4-d]thiazole Dibromide (Bz2TTz2+). Py2TTz (0.1 g, 0.34 mmol) was heated to 130 °C for 6 h in 3 mL of benzyl bromide. Hexanes were added to the cooled solution to form a suspension, which was filtered and rinsed with additional hexanes to afford 0.081 g (50%) of a bright yellow solid. 1H NMR (300 MHz, CD3CN, TMS, δ): 8.89 (d, J = 6.87 Hz, 4H), 8.54 (d, J = 6.9 Hz, 4H), 5.78 (s, 4H), 7.52 (m, 10H). 13C NMR (126 MHz, d6‑DMSO, TMS, δ): 165.77, 155.69, 146.81, 146.49, 134.72, 130.02, 129.84, 129.41, 125.17, 63.73. UV−vis λmax (H2O, ε = M−1cm−1): 395 nm (ε = 46 000) ESI-MS: calcd for C28H22N4S22+, 239.0637; found, 239.0655. N,N′-Dimethyl 2,5-Bis(4-pyridinium)thiazolo[5,4-d]thiazole Ditosylate (Me2TTz2+). Py2TTz (0.1 g, 0.34 mmol) was warmed to 30 °C for 48 h in 3 mL of methyl p-tosylate. The precipitate was collected, washed with hexanes, and dried to afford 0.072 g (65%) of a bright yellow solid. 1H NMR (300 MHz, CD3CN, TMS, δ): 8.75 (d, J = 6.87 Hz, 4H), 8.54 (d, J = 6.87 Hz, 4H), 4.34 (s, 6H). 13C NMR (126 MHz, d6‑DMSO, TMS, δ): 165.75, 155.49, 147.32, 146.06, 124.28, 48.45. UV−vis λmax (H2O, ε = M−1cm−1): 390 nm (ε = 32 000) ESI-MS: calcd for C16H14N4S22+, 163.0324; found, 163.0445. N,N′-Dioctyl 2,5-Bis(4-pyridinium)thiazolo[5,4-d]thiazole Dibromide (Oct2TTz2+). Py2TTz (0.155 g, 0.52 mmol) was heated to 190 °C for 6 h in 3 mL of bromooctane under N2. Solution was cooled and added to 10 mL of CH3OH, which was washed several times with

Scheme 1. Synthesis of N,N′-Dialkylated/Dibenzylated 2,5Bis(4-pyridinium)thiazolo[5,4-d]thiazolea

a

(i) 150°C in DMF, 6h (ii) heat in MeTos, C8H17Br, or C7H7Br.

thiazolo[5,4-d] thiazole are highly fluorescent materials with a unique fused-thiazole bicyclic aromatic structure. The thiazolo[5,4-d]thiazole (TTz) synthesis proceeds through the double condensation of an aromatic aldehyde with dithiooxamide in air or with a chemical oxidant to form a thermooxidatively stable compound.31−33 The TTz synthesis produces the bicyclic product with little partial condensation of the aromatic aldehydes with dithiooxamide simplifying isolation and purification. The TTz moiety has relatively high fluorescence emission intensity due to its rigid aromatic structure. It has also demonstrated very high free charge carrier mobilities (>3 cm2 V−1 s−1) when incorporated into various conjugated polymers and devices.34−36 The ease of synthesis of the TTz fused heterocyclic ring system and its successful use in recent solar/ optoelectronic applications makes this bicyclic group an ideal πconjugated electron acceptor material.37−40 In this report, we characterize the fluorescence dynamics, electrochemical, and electrochromic properties of N,N′-dialkyl or N,N′-dibenzyl 2,5-bis(4-pyridinium)thiazolo[5,4-d]thiazole (TTz2+) derivatives. We show that alkylation, benzylation, or protonation of the 4-dipyridyl peripheral groups of the TTz structures increase the fluorescence quantum from (ΦF) 0.22 to 0.96 and accompanies low reversible reduction potentials and electrochromism similar to traditional viologen structures. We also show that the fluorescence of the 1 and 2 e− reduced TTz derivatives is deactivated, making these materials useful fluorescence sensors for studying molecular electron transfer dynamics. The multifunctional nature of these new materials and their ease of preparation make them important systems for fundamental studies into photoinduced electron transfer and applications that pair molecular electrochromism with strong fluorescence emission.

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EXPERIMENTAL SECTION

Materials and Instrumentation. Materials for N,N′-dialkylated/ N,N′-dibenzylated dipyridinium thiazolo[5,4-d]thiazole syntheses: 4pyridine carboxaldehyde, dithiooxamide, 1-bromooctane, acetonitrile (ACN), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ammonium hexafluorophosphate (NH4PF6), and methyl tosylate were all purchased from either Sigma-Aldrich or TCI America and used without further purification. 1H NMR measurements were obtained with a JEOL 300 or 500 MHz NMR. UV−vis spectra were collected on a Cary 300 UV−vis spectrophotometer for solution measurements. UV−vis spectra for electrochemically reduced TTz2+ species were obtained using a spectroelectrochemical cell with a 1.0 mm path and a printed platinum honeycomb working/counter electrode, and an Ag/ AgCl reference electrode. Potentials were applied for 30 s before UV− 8468

DOI: 10.1021/jacs.7b01005 J. Am. Chem. Soc. 2017, 139, 8467−8473

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Journal of the American Chemical Society hexanes. CH3OH was removed to afford 0.184 g (61%) of an orange solid. 1H NMR (300 MHz, CD3CN, TMS, δ): 8.91 (d, J = 6.96 Hz, 4H), 8.58 (d, J = 6.96 Hz, 4H), 4.61 (t, J = 7.52 Hz, 4H), 1.95 (m, 4H), 1.38 (m, 20H), 0.89 (t, 6H). 13C NMR (126 MHz, CD3CN, TMS, δ): 165.12, 155.91, 147.09, 145.71, 124.65, 61.77, 31.49, 31.02, 28.79, 28.66, 25.63, 22.40, 13.43. UV−vis λmax (H2O, ε = M−1cm−1): 390 nm (ε = 32 000) ESI-MS: calcd for C30H42N4S22+, 261.1434; found, 261.1682 .

SI). The crystal structure shows the highly planar TTz core with peripheral dibenzyl groups attached to the 4-pyridinium groups. Both bromides are shown interacting with TTz ring systems. The TTz bond lengths and the nearly planar dihedral angle (3.7°) between the pyridinium groups and TTz core are in agreement with reported TTz crystal structure derivatives.47,57,62 In contrast, MV2+ structures indicate dihedral angles of 37° between pyridinium rings in the solid state.63 The crystal-packing diagram shows Bz2TTz2+ molecules orientated orthogonal to one another with the peripheral benzyl groups stacked planar between two Bz2TTz2+ molecules. In aqueous solution, the alkylated TTz2+ derivatives show nearly identical UV−vis spectra with an absorption peak between 378−395 nm (ε = 40−50 × 103 M−1·cm−1) representing the S0−S1 singlet transition (π−π* transition) (Figure 2). Peak fluorescence emission occurs between 452−

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RESULTS AND DISCUSSION Synthesis and Photophysical Characterizations. The bicyclic thiazolothiazole moiety has become increasingly popular for integrating into molecular optoelectronic applications such as fluorescent sensors,46 field-effect transistors,34,47−49 and in light harvesting dyes for molecular photovoltaic devices.40,50−54 The TTz rigid structure and high thermooxidative stability make it especially attractive for dyes and conjugated polymers for solar energy conversion.31,49,54 New methods to form the bicyclic fused TTz ring system have also been developed,33,55,56 offering several new synthetic strategies and access to new complexes.44,57 Novel metal organic framework structures (MOFs) and 2D polymer sheets have demonstrated its usefulness as a rigid, aromatic, and electroactive framework ligand.43,58−60 Formation of the TTz structure by condensation of aromatic aldehydes and dithiooxamide is widely used,31,61 and was utilized in this work to synthesize the 2,5-bis(4-pyridyl)thiazolo[5,4-d]thiazole (Py2TTz) starting material (Scheme 1). Dialkylation proceeded by heating Py2TTz in 1-bromooctane, benzyl bromide, or methyl tosylate under neat conditions. H2TTz2+ was prepared by dissolving Py2TTz in 0.03 M HCl, which avoids protonation of the TTz core.36 N-quaternization was confirmed by monitoring the shift in the α-pyridyl protons from 7.9 to 8.5/8.6 ppm in the 1H NMR spectrum. The dialkylated and dibenzylated TTz2+ derivatives were readily solubilized in methanol, which was used to remove unreacted Py2TTz. Py2TTz was found to be sparingly soluble in solvents such as chloroform and dichloromethane. Bz2TTz2+ and Me2TTz2+ were both highly soluble in water, while the Oct2TTz2+ was only partially soluble. Single crystals of the dibenzyl derivative Bz2TTz2+ suitable for X-ray crystallography were obtained by slow evaporation of an aqueous solution (Figure 1 and Tables S2−S6 of the Supporting Information,

Figure 2. Normalized absorption and emission spectra of H2TTz2+, Me2TTz2+, Oct2TTz2+, and Bz2TTz2+ (inset: fluorescence lifetime decay of H2TTz2+ and Bz2TTz2+, and below: image of 10 mM solution of Bz2TTz2+ without and with UV light illumination).

461 nm. Strong emission could be observed with UV illumination under room light conditions (Figure 2−inset). Variations of peripheral substituent (methyl, octyl, benzyl) slightly red-shifted the absorbance and emission maxima. DFT calculations indicate electron density delocalized across the coplanar TTzperipheral pyridyl groups (with and without alkylation/protonationFigures S1−S5) in the LUMO and LUMO+1. More of electron density is localized around TTz core in the HOMO ground state levels (for HOMO−H2TTz2+ and Me 2TTz2+, HOMO-3−Oct2 TTz2+, and HOMO-2− Bz2TTz2+). Previously reported TTz-based structures show fluorescence quantum yields (ΦF) between 0.16−0.25 for diphenyl TTz,36 and 0.23−0.43 for a TTz based chemosensor.46 Fluorescence of the Py2TTz also showed a similar quantum yield of ΦF = 0.22 (Table S1). Upon protonation or alkylation, the quantum yield rose dramatically to ΦF = 0.96 for the H2TTz2+ derivative and ΦF = 0.92 for the Me2TTz2+ derivative. The Bz2TTz2+ and Oct2TTz2+ derivatives showed slightly lower quantum yields of 0.79 and 0.87, respectively (Table 1). Similar enhancements in fluorescence were observed with 2- and 3-aminopyridines in strong acid, although the 4-aminopyridines show low fluorescence yields due to excited-state charge transfer effects.64 By comparison, the highest quantum yields of MV2+ were ΦF =

Figure 1. Crystal structure a) and packing diagram for Bz2TTz2+ b). 8469

DOI: 10.1021/jacs.7b01005 J. Am. Chem. Soc. 2017, 139, 8467−8473

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Journal of the American Chemical Society Table 1. Summary of Photophysical and Electrochemical Properties of Thiazolothiazole Viologens λmax (nm) (ε = M−1cm−1) Py2TTz H2TTz2+ Me2TTz2+ Oct2TTz2+ Bz2TTz2+ a

357 368 390 390 395

(32 000) (46 000) (32 000) (32 000) (46 000)

τF (ns)b 1.09 2.41 2.14 1.9 1.79

± ± ± ± ±

0.20 0.02 0.16 0.21 0.06

(ΦF)c

Ered1d (V vs SCE)

Ered2e (V vs SCE)

± ± ± ± ±

−0.52 −0.53 −0.46

−0.58 −0.57 −0.50

0.22 0.96 0.92 0.87 0.79

0.03 0.03 0.02 0.02 0.04

a

Measurements for H2TTz2+ obtained in aqueous acidic conditions (Py2TTz in a 30 mM HCl solution). bSinglet PL(t) decays−mono exponential fit (average of 3 samples) measured in H2O (1.0 μM)−TTz2+(Br−)2 salts. cCalculated using comparative method (9,10-diphenylanthracene)64 (average of 3 samples)−TTz(Br−)2 salts. d,eRedox potentials measured vs SCE reference electrode and measured in 0.1 M (TBAH) degassed DMSO solutions.

0.03 in ACN, and little or no fluorescence in water.65 Studies of the photochemical dynamics of MV2+ in various solvents have demonstrated ultrafast MV2+ photoreduction in methanol with fluorescence lifetimes (τF) of 1 ns in acetonitrile.65,66 Interestingly, additional work has shown that MV2+ responds as a photoacid with the excited state rapidly forming a MV2+− hydroxide ion pair in aqueous solution.67,68 The TTz2+ derivatives showed long-lived singlet fluorescence lifetimes (τF) in water ranging between 1.8 and 2.4 ns (Table 1). This was on average more than double the fluorescence lifetime of the Py2TTz starting material. The PF6− salts of the alkylated derivatives had similar fluorescence lifetimes in ACN (Table S1). The protonated derivative exhibited the longest fluorescence lifetime decay τF = 2.4 ns. The rate of fluorescence (kF = ΦF/τF) of the diprotonated TTz2+ was slightly lower (kF = 3.98 × 108 s−1) with the alkylated derivatives showing kF values between 4.3 × 108 s−1 and 4.4 × 108 s−1. The high fluorescent yields and long PL decays suggest that the derivatives have stable excited states under ambient conditions in water, and low rates of intersystem crossing to reactive triplet states.69 Aqueous solutions maintained their strong fluorescence for extended periods (>1 yr) under ambient laboratory conditions. Electrochemical and Electrochromic Characterization. Cyclic voltammetry (CV) characterizations of the dialkylated/ dibenzylated Py2TTz materials in DMSO show two reversible, closely spaced reductions occurring between −0.46 and −0.58 V vs SCE (Table 1 and Figure 3). The Me2TTz2+(PF6−)2 derivative indicates two reversible reduction peaks in the cyclic voltammogram (Figure 3a). The

two reduction peaks (E°1, red and E°2,red) are more clearly seen in the square wave voltammograms for each TTz compound (Figures 3c, S6, and S7, and Table 1). The trend observed for the alkylated TTz derivatives showed E°1, red reduction potentials in the order Oct2TTz2+ < Me2TTz2+ < Bz2TTz2+ following a similar trend seen in methyl, octyl, and benzyl viologen derivatives.5 The first reduction of the Me2TTz2+ derivative is considerably less negative than methyl viologen (MV2+ = −0.7 V vs SCE).27 The δEred of closely spaced reductions varied slightly among the derivatives with Me2TTz2+ exhibiting the largest δEred ≈ 60 mV while both Bz2TTz2+ and Oct2TTz2+ showed δEred ≈ 40 mV. Previous work on extended viologens containing a single thiophene, furan, phenyl, or vinyl spacer show two one-electron reductions,70 while recent work using a bithiophene linked viologen exhibits just a single twoelectron reduction process.71−73 The TTz2+ electrochemical characterization suggests that the thiazolo[5,4-d]thiazole provides good electronic communication between the N,N′dialkylated/dibenzylated pyridinium rings. The TTz2+ derivatives showed good electrochemical stability in DMSO with similar CVs obtained after several weeks in solution under ambient laboratory conditions. Spectroelectrochemical measurements were acquired by monitoring the UV−vis absorbance spectra during electrolysis of degassed TTz2+ solutions (Figures 4a,b, S8, and S9). Figure 4a shows the spectroelectrochemical data of the TTz2+(PF6−)2 derivative in ACN, sweeping from −0.14 V to −0.65 V vs Ag/ AgCl. Applied potentials from −0.14 V to −0.41 V shows a decrease in the absorption band at 395 nm, and an increase at 616 nm. From electrochemical data, this light blue species is assigned the TTz1+ radical cation. Upon further reduction (−0.41 V to −0.65 V), a strong absorption band grows in at 672 nm, with additional bands below 300 nm (∼250 nm). This is assigned the diradical TTz viologen species (a darker blue solution). These spectroelectrochemical features were also observed with the Me2TTz2+ and Oct2TTz2+ derivatives (Figures S8 and S9). The electrochromic reduction is reversible, and the initial TTz2+ spectrum can be obtained by reversing the applied voltage back to −0.14 V (Figure 4b). To visually demonstrate the electrochromism of TTz2+ series, propylene carbonate (PC) solutions of the derivatives with TBAH supporting electrolyte were introduced between two transparent fluorine-doped tin oxide conductive oxide (FTO) electrodes, which were sandwiched and held together with a 0.5 mm adhesive spacer (Figure 4c). The solution was reduced and rapidly activated the TTz2+ electrochromic window which could be reversibly cycled several times before building up a significant concentration of the fully reduced TTz.13 Shorting the electrochromic window or switching to a positive applied voltage allows for the dark blue, reduced TTz

Figure 3. Cyclic voltammograms (CVs) of Me2TTz2+(PF6−)2 at 50, 100, 150, 200, 250, 300 mV s−1 in DMSO vs SCE reference a), proposed Me2TTz2+ structures during 1 and 2 e- reductions b), and square wave voltammogram of Me2TTz2+(PF6−)2 (net and forward/ reverse) c). 8470

DOI: 10.1021/jacs.7b01005 J. Am. Chem. Soc. 2017, 139, 8467−8473

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Journal of the American Chemical Society Figure 4. continued

fluorescence spectral changes during bulk electrolysis of a degassed Bz2TTz2+ solution c).

species to be converted back to the clear/light yellow TTz2+ species. The bright blue TTz2+ fluorescence ∼455 nm was observed before and after cycling the electrochromic window (Figure 4c) and significantly decreased when illuminated in its reduced state (a mix of both TTz1+ and TTz neutral species). Some fluorescence was still detectable in the reduced state due to the thickness of the electrochromic window. To observe the fully quenched fluorescence of the reduced TTz form, the fluorescence intensity was monitored during bulk electrolysis of an ACN solution (Figure 4C). The fluorescence intensity is strongly quenched after applying −0.7 V for to a solution of the Bz2TTz2+ derivative for 25 min. Applications of modulating both fluorescence intensity and color tuning using electrofluorochromism is a potential application for these materials.74−76 Several systems have been explored including a bithiophene-linked extended viologen.72,73 The strong sensitivity of the fluorescence to the reduced TTz state, suggests that these compounds could also serve as efficient fluorescent sensors for detecting electron transfer dynamics.30 This would be especially useful when tracking photoinduced electron transfer (PET) events,23 where these TTz2+ materials could provide fluorescence and photochemical monitoring from both donor and acceptor species.

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CONCLUSIONS This study reports on the fluorescence properties and electrochemical characteristics of a series of alkylated/ benzylated thiazolo[5,4-d]thiazole based viologens. Aqueous solutions of the diprotonated, dimethyl, dioctyl, and dibenzyl derivatives show stable/high fluorescence quantum yields and long-lived fluorescence lifetimes in water and acetonitrile. The derivatives also exhibit vibrant and reversible electrochromic properties with reduction potentials that are more positive than methyl viologen. The planar thiazolothiazole backbone exhibits good electronic communication as evidenced by two reduction peaks. These compounds show their usefulness as multifunctional, π-conjugated electron acceptor materials. The strong fluorescence emission can be reversibly “turned off” by engaging the electrochromic properties under negative applied bias. The strong emission and the ability to suppress the fluorescence upon reduction can enable the use of these fluorescent TTz2+ materials for sensing photoinduced electron transfer processes. With increased usage of methyl viologen and related derivatives for solar energy conversion, electrochromism, redox-flow batteries, electron transfer sensing, and biological studies, it is likely that the properties observed with this series of highly fluorescent TTz chromophores will find use in a wide variety of new and creative photochemical applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 4. Spectroelectrochemistry of 50 μM Bz2TTz2+(PF6−)2 in ACN, forward sweep (−0.14 to −0.65 V vs Ag/AgCl) a), and reverse sweep with each potential applied for 30 s b). Image of neutral and reduced Oct2TTz2+/1 mM propylene carbonate solutions encased in a glass FTO electrode window in room light (above) and under UV illumination (below), with the proposed reduced TTz2+ structures, and

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01005. Fluorescence data (PDF), DFT calculations, cyclic and square wave voltammetry, spectroelectrochemical data (PDF) 8471

DOI: 10.1021/jacs.7b01005 J. Am. Chem. Soc. 2017, 139, 8467−8473

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Journal of the American Chemical Society

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X-ray crystallographic data for Bz2TTz2+ (CCDC # 1494013) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Michael G. Walter: 0000-0002-9724-265X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Department of Chemistry at the University of North Carolina at Charlotte and the B.S. Chemistry program. M.G.W. acknowledges ESI/MS instrumentation research support from the NSF (CHE 1337873). S.R.H. acknowledges support from the Charlotte Research Scholars (2013). A.N.W. and N.A.S acknowledges support from the NSF-REU site program in partnership with the ASSURE program of the DoD (CHE 1156867). M.G.W. acknowledges assistance from David Marin, Madison Kendrick (Army Education Outreach ProgramREAP), and Pokyes Kromtit (ACS Project SEED student). M.G.W. is also grateful to Professor Thomas Schmedake (UNC Charlotte) for assistance with the X-ray crystallographic data.

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Article

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NOTE ADDED AFTER ASAP PUBLICATION A partially corrected version of this paper was published on June 2, 2017. The fully corrected version was reposted on June 7, 2017.

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DOI: 10.1021/jacs.7b01005 J. Am. Chem. Soc. 2017, 139, 8467−8473