Visible Photosensitization of trans-Styrylpyridine Coordinated to fac

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Visible Photosensitization of trans-Styrylpyridine Coordinated to fac-[Re(CO)3(dcbH2)]+: New Insights Lais S. Matos, Ronaldo C. Amaral, and Neyde Y. Murakami Iha* Laboratory of Photochemistry and Energy Conversion, Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Avenida Prof. Lineu Prestes 748, 05508-000 São Paulo, São Paulo, Brazil

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S Supporting Information *

ABSTRACT: A strategic methodology has been developed to effectively synthesize the fac-[Re(CO)3(dcbH2)(trans-stpy)]+ complex, where dcbH2 = 2,2′-bipyridine-4,4′-dicarboxylic acid and trans-stpy = trans-4-styrylpyridine, which has been designed to efficiently absorb visible light. The complex exhibits outstanding trans-to-cis photoisomerization with 436 nm irradiation (Φtrans→cis = 0.50 ± 0.03), in contrast to the photochemical behavior previously reported in the literature (Faustino, L. A.; et al. Inorg. Chem. 2018, 57, 2933−2941). The main emphasis here is to address the synthetic strategy for obtaining the actual complex, its characterization, and an accurate description of its photochemical and photophysical behavior, which reveal new insights into this complex.

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fac-[Re(CO)3(dmcb)(trans-stpy)]+ and fac-[Re(CO)3(dmcb)(trans-stpyCN)]+ [dmcb = 4,4′-dimethoxycarbonyl-2,2′-bipyridine, trans-stpy = trans-4-styrylpyridine, trans-stpyCN = trans4-(4-cyano)styrylpyridine], which are capable of initiating photoisomerization in a lower-energy spectral region.38 Here, we report a strategy for the synthesis of fac[Re(CO)3(dcbH2)(trans-stpy)]+, where dcbH2 = 2,2′-bipyridine-4,4′-dicarboxylic acid, and its photochemical behavior in achieving sensitization at up to the 436 nm irradiation. Recently, Faustino et al.1 reported a similar investigation but without photoisomerization in the visible, giving low efficiencies with 404 nm irradiation. The main goal of this paper is to discuss a judicious synthetic route and strategy for the synthesis of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ along with a step-by-step 1H NMR characterization procedure and a study of its true photochemical behavior.

INTRODUCTION The photoisomerization of rhenium(I) carbonyl complexes with stilbene-like ligands has been investigated extensively,2−39 with a special focus on compounds that extend their absorption to the visible to create photochemical and photophysical properties suitable for applications in molecular devices.3,22,33,34,37,40 These studies, and an understanding of their properties, allow modulation and control of key roles into the development of photoinduced devices, such as photosensitizers, photoswitches, and photoresponsive devices.22,32,40−45 Photochemical processes of stilbenes themselves have also attracted significant attention.46−54 trans-to-cis photoisomerization occurs, in general, by singlet excited states (S0 → S1 or S0 → Sn, π → π*) that promote a double-bond twist,55 typical of motion in molecular machines.56−60 Similar photoactivation, still preserving the main photochemical characteristic of stilbenes, can be achieved at lower energies when it is coordinated to rhenium(I) tricarbonylpolypyridyl complexes, fac-[Re(CO)3(NN)(trans-L)]+, where NN = polypyridyl and L = stilbene-like ligands.22,40−45 Coordination in the complex facilitates electron and/or energy transfer because of the high spin−orbit coupling and extends absorption into the visible.42,43 Coordination also improves the versatility and variety of synthetic tools that can be conveniently employed to design new compounds with desired characteristics to tune excitedstate properties through changes in polypyridyl and photoisomerizable ligands.22 As a result of a series of studies, the best results reported so far achieved up to 404 nm sensitization, but usually with a lower quantum yield.24,27,30−32,42,45 Only recently, our group succeeded in synthesizing new complexes © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Materials. All chemicals employed in the syntheses (reagent grade) and solvents for photochemical and photophysical measurements (Aldrich, HPLC grade) were used as supplied. Syntheses of Ligands. trans-4-Styrylpyridine (trans-stpy). The trans-stpy ligand was synthesized with modifications of a procedure previously described.20,30,31,37,39 Benzaldehyde (7.0 mL, 40 mmol, Aldrich) and 4-picoline (4.0 mL, 41 mmol, Aldrich) were refluxed for 12 h in acetic anhydride (4.3 mL, 46 mmol, Merck). The reaction was monitored by UV−vis spectroscopy and thin-layer chromatography (TLC). The solution was distilled and the residual oil was dropped in water to yield a light-yellow precipitate. The crude product was Received: May 13, 2018

A

DOI: 10.1021/acs.inorgchem.8b01304 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

was dried under vacuum (10 mmHg, 60 °C) to obtain 3.15 g of the pure product (5.72 mmol, 90% yield). 1H NMR (300 MHz, DMSOd6): δ 9.21 (d, 2H, J3 = 5.8 Hz), 9.14 (s, 2H), 8.13 (d, J3 = 5.7 Hz). fac-[Re(CO)3(dcmb)Cl]. The fac-[Re(CO)3(dmcb)Cl] complex was synthesized according to a procedure previously described.8,9,39 The [Re(CO)5Cl] (2.01 g, 5.55 mmol, Aldrich) precursor and an excess of dmcb (1.51 g, 5.55 mmol) were suspended in 70 mL of xylene (Synth) and heated to reflux for 5 h. The reaction was followed by UV−vis spectroscopy and TLC. After cooling to room temperature, the resulting solid was separated by filtration and washed with xylene. The crude product was recrystallized from dichloromethane (Synth) by the slow addition of n-pentane (Synth). The product was dried under vacuum (10 mmHg, 60 °C) to obtain 2.20 g of the pure product (5.29 mmol, 95% yield). 1H NMR (300 MHz, CD3CN): δ 9.19 (dd, J3 = 5.7 Hz, J5 = 0.8 Hz, 2H), 8.95 (dd, J4 = 1.7 Hz, J5 = 0.8 Hz, 2H), 8.07 (dd, J3 = 5.7 Hz, J4 = 1.7 Hz, 2H), 4.02 (s, 6H). Anal. Calcd for C17H11ClN2O7Re: C, 35.33; H, 2.09; N, 4.85. Found: C, 35.39; H, 2.09; N, 4.65. fac-[Re(CO)3(dcbH2)(CD3CN)]+. A total of 0.24 g (0.43 mmol) of fac-[Re(CO)3(dcbH2)Cl] was suspended in 35 mL of tetrahydrofuran (THF; Synth), and 0.11 g (0.43 mmol) of silver trifluoromethanesulfonate (Agtfms; Aldrich) was added to the mixture under an argon atmosphere. After 2 h, AgCl was removed by filtration. Then, the solution was rotovapped until the formation of an oil, which was solubilized in CD3CN to obtain the fac-[Re(CO)3(dcbH2)(CD3CN)]+ complex. 1H NMR (300 MHz, CD3CN): δ 9.32 (d, J3 = 5.7 Hz, 2H), 9.14 (s, 2H), 8.29 (dd, J3 = 5.7 Hz, J4 = 1.6 Hz, 2H). fac-[Re(CO)3(dmcb)(trans-stpy)]PF6. The fac-[Re(CO)3(dmcb)(trans-stpy)]PF6 complex was synthesized according to a procedure previously described for fac-[Re(CO)3(dmcb)(Etpy)]PF6, with slight modifications.39,63 The fac-[Re(CO)3(dcmb)Cl] precursor (0.77 g, 1.3 mmol) was added in 25 mL of THF (Synth). The suspension was maintained under stirring and an argon atmosphere for ∼1 h; then, 0.34 g (1.3 mmol) of Agtfms (Aldrich) was added and maintained under reflux and stirring for 4 h. The reaction was followed by UV− vis spectroscopy and TLC. The precipitated AgCl was collected by filtration, and the solution was transferred to another flask and maintained under stirring and an argon atmosphere for ∼30 min; then, 0.37 g (2.0 mmol) of trans-stpy was added. The mixture remained under reflux for 12 h, and the reaction was monitored via both absorption spectroscopy and TLC. At room temperature, 1.1 g (6.5 mmol, Aldrich) of NH4PF6 was added, and the mixture was rotovapped until the formation of a red oil and precipitated by adding 50 mL of ethanol. The excess of the trans-stpy ligand was removed by suspension of the solid in ethanol (Synth). The solid was filtered off, washed with ultrapure water and ethyl ether (Synth), and dried under vacuum (10 mmHg, 60 °C) to obtain 0.78 g (0.89 mmol, 67% yield) of the pure product. 1H NMR (300 MHz, CD3CN): δ 9.40 (dd, J3 = 5.7 Hz, J5 = 0.7 Hz, 2H), 8.90 (dd, J4= 1.6 Hz, J5 = 0.7 Hz, 2H), 8.22 (dd, J3 = 5.7 Hz, J4 = 1.7 Hz, 2H), 8.10 (dd, J3 = 5.4 Hz, J3 = 1.4 Hz, 2H), 7.56 (dd, J3 = 7.5 Hz, J3 = 1.9 Hz, 2H), 7.45 (d, J3 = 16.4 Hz, 1H), 7.37 (m, 5H), 7.05 (d, J3 = 16.4 Hz, 1H), 4.02 (s, 6H). Anal. Calcd for C30H23F6N3O7PRe: C, 41.48; H, 2.67; N, 4.84. Found: C, 41.50; H, 2.70; N, 4.59. fac-[Re(CO)3(dcbH2)(trans-stpy)]PF6. The complex was obtained by hydrolysis by dissolving fac-[Re(CO)3(dmcb)(trans-stpy)]PF6 (0.15 g, 0.17 mmol) in 10.2 mL of methanol (Synth). Then, 1.7 mL of a 0.40 mol L−1 K2CO3 (Aldrich) aqueous solution was added, and the solution was maintained under stirring for 30 min. The reaction was monitored by TLC, and after 30 min, 92 μL (1.0 mmol) of a 60 wt % HPF6 aqueous solution (Aldrich) was added. The mixture was rotovapped, and the solid obtained was suspended in ultrapure water, filtered off, washed with water and ethyl ether (Synth), and dried under vacuum to obtain 0.11 g (0.13 mmol, 76% yield) of the pure product. HRMS (ESI-TOF, CH3CN, [M]+). Calcd for C28H19N3O7Re: m/z 696.0781. Found: m/z 696.0786. 1H NMR (200 MHz, CD3CN): δ 9.29 (s, 2H), 9.27 (d, J3 = 6.0 Hz, 2H), 8.15 (d, J3 = 6.0 Hz, 2H), 8.08 (d, J3 = 6.4 Hz, 2H), 7.52 (m, 2H), 7.45 (d, J3 = 16.4 Hz, 1H), 7.35 (m, 5H), 7.02 (d, J3 = 16.4 Hz, 1H), 2.38

recrystallized from ethanol by the slow addition of ultrapure water. The solid was filtered off, washed with ultrapure water and ethyl ether (Synth), and dried under vacuum (10 mmHg, 60 °C) to obtain 3.73 g of the pure product (20.6 mmol, 51% yield). 1H NMR (CDCl3, 300 MHz): δ 8.57 (d, 2H, J3 = 5.8 Hz), 7.53 (d, 2H, J3 = 7.4 Hz), 7.44− 7.28 (m, 6H), 7.00 (d, 1H, J3 = 16.3 Hz). 1H NMR (CD3CN, 300 MHz): δ 8.53 (dd, 2H, J3 = 6.2 and 1.7 Hz), 7.62 (d, 2H, J3 = 7.0 Hz), 7,40 (m, 6H), 7.16 (d, 1H, J3 = 18.0 Hz). 13C NMR (CD3CN, 75 MHz): δ 150.12, 144.50, 136.06, 133.07, 128.77, 128.67, 126.93, 125.91, 120.76. Anal. Calcd for C13H11N·1/10H2O: C, 85.31; H, 6.17; N, 7.65. Found: C, 85.41; H, 5.90; N, 7.49. Protonated trans-stpy (trans-Hstpy+). The protonation of transstpy best describes the coordination effects and was obtained by adding HCl (400 μL, 10 mol L−1, Synth) to a saturated solution of trans-stpy (0.380 g, 2.33 mmol) to yield a pure yellow precipitate. The reaction was monitored by UV−vis spectroscopy and TLC. The solid was filtered off, washed with ultrapure water and acetonitrile (Synth), and dried under vacuum (10 mmHg, 60 °C) to obtain 0.34 g of the product (1.8 mmol, 76% yield). 1H NMR (CD3CN, 300 MHz): δ 8.56 (d, J3 = 6.8 Hz, 2H), 8.00 (d, J3 = 6.8 Hz, 2H), 7.90 (d, J3 = 16.4 Hz, 1H), 7.70 (d, J3 = 7.8 Hz, J4 = 2.3 Hz, 2H), 7.46 (m, 3H), 7.23 (d, J3 = 16.4 Hz, 2H). Anal. Calcd for C13H12ClN·5/7H2O: C, 67.72; H, 5.87; N, 6.08. Found: C, 67.74; H, 5.70; N, 5.81. 2,2′-Bipyridine-4,4′-dicarboxylic acid (dcbH2). The dcbH2 ligand was synthesized according to a procedure previously described, with slight modifications.61 A total of 3.02 g (16.4 mmol) of 3,3-dimethyl1-butanol was slowly added to a solution of K2Cr2O4 (9.70 g, 33.0 mmol, Vetec) in 98% H2SO4 (40.0 mL, Mallinckrodt) under magnetic stirring at room temperature. The resultant orange slurry became dark green, and the reaction was interrupted after 30 min. The mixture was then poured into 100 mL of cold water, forming a light-yellow precipitate. After filtration and drying, the solid was recrystallized to isolate the desired compound free of Cr3+ ions by dissolution in an 40% alkaline aqueous NaOH (Aldrich) solution, followed by slow acidification (pH = 2) with a 10% aqueous HCl solution (Synth). The solid was filtered, washed with ultrapure water and ethyl ether (Synth), and then dried under vacuum (10 mmHg, 60 °C) to obtain 3.51 g (14.4 mmol, 88% yield) of the pure dcbH2 product. 1H NMR (300 MHz, ND4OD): δ 8.98 (d, J3 = 4.8 Hz, 2H), 8.61 (s, 2H), 7.69 (dd, J3 = 5.1 Hz, J4 = 1.5 Hz, 2H). 13C NMR (75 MHz, ND4OD): δ 172.59, 155.66, 149.81, 146.52, 123.53, 121.23. Anal. Calcd for C12H8N2O4·1/4H2O: C, 57.95; H, 3.44; N, 11.26. Found: C, 57.88; H, 3.45; N, 11.37. 4,4′-Dimethoxycarbonyl-2,2′-bipyridine (dmcb). The dmcb ligand was synthesized with modifications of a procedure previously described.62 A solution containing 3.11 g (12.5 mmol) of dcbH2 in 60 mL of methanol (Synth) was added to 10 mL of concentrated H2SO4 (98%, Mallinckrodt). The system was placed in the oven of a microwave apparatus, and then it was irradiated for 4 h (reflux mode, 25 W) and monitored by UV−vis spectroscopy and TLC. A total of 100 mL of ultrapure water was added to the mixture, and the pH was adjusted to 10 with 4 mol L−1 aqueous NaOH (Aldrich). The solid was filtered off, washed with ultrapure water and ethyl ether (Synth), and dried under vacuum (10 mmHg, 60 °C) to obtain 3.31 g (12.2 mmol, 97% yield) of the pure dmcb product. 1H NMR (300 MHz, CDCl3): δ 8.93 (s, 2H), 8.84 (d, J3 = 5.0 Hz, 2H), 7.88 (dd, J3 = 5.0 Hz, J4 = 1.5 Hz, 2H), 3.97 (s, 6H). 13C NMR (75 MHz, CDCl3): δ 165.82, 156.57, 150.35, 138.79, 123.46, 120.77, 52.98. Anal. Calcd for C14H12N2O4: C, 61.73; H, 4.44; N, 10.44. Found: C, 61.61; H, 4.33; N, 10.03. Syntheses of Complexes. fac-[Re(CO)3(dcbH2)Cl]. The fac[Re(CO)3(dcbH2)Cl] complex was synthesized according to a procedure previously described for fac-[Re(CO)3(bpy)Cl], with slight modifications.8,9 The [Re(CO)5Cl] (2.30 g, 6.36 mmol) precursor and an excess of dcbH2 (1.71 g, 7.00 mmol) were suspended in 50 mL of xylene (Synth) and heated to reflux for 8 h. The reaction was monitored by UV−vis spectroscopy and TLC. After cooling to room temperature, the resulting solid was separated by filtration and washed with xylene. The crude product was recrystallized from methanol (Synth) by the slow addition of ethylic ether (Synth). The product B


Article

Inorganic Chemistry Scheme 1. Synthetic Methodology for Obtaining fac-[Re(CO)3(dcbH2)(trans-stpy)]PF6

Table 1. 1H NMR Data for fac-[Re(CO)3(dmcb)Cl], trans-stpy, and fac-[Re(CO)3(dmcb)(trans-stpy)]+ in CD3CN at 298 K

a

300 MHz 1H NMR. b200 MHz 1H NMR (Figure S4).

(br). Anal. Calcd for C28H19F6N3O7PRe: C, 40.01; H, 2.28; N, 5.00. Found: C, 40.31; H, 2.13; N, 4.85. General Procedures. UV−Vis Absorption. Electronic absorption spectra were recorded on a Hewlett-Packard 8453 spectrophotometer with 1.000- or 0.100-cm-optical-length quartz cuvettes. Microwave Reaction. Microwave reaction was performed on a CEM Discover Synthesis Unit (CEM Co.), with a continuously focused microwave power delivery system in a glass vessel (100 mL) under magnetic stirring. 1 H NMR. 1H NMR spectra were recorded on a Bruker Advance III (200 MHz), a Varian Unity Inova (300 MHz), or a Bruker AIII (800 MHz) spectrometer at 298 K using CD3CN, CDCl3, ND4OD, or DMSO-d6 as the solvent, having 1.94 ppm (CD3CN), 7.27 ppm (CDCl3), 4.75 ppm (ND4OD), and 0.00 ppm (TMS), respectively, as the internal standard. Mass Spectrometry. High-resolution mass spectroscopy spectra were recorded on a Bruker Daltonics MAXIS 3G electrospray ionization time-of-flight (ESI-TOF) spectrometer. CHN. Elemental analysis was perfomed using a PerkinElmer 2400 series II analyzer. Photochemical Measurements. Photolyses of solutions at 313, 334, 365, 404, or 436 nm were carried out as previously described,14,30 using 200 W or 500 W mercury (xenon) Oriel systems by selecting the wavelength with the appropriate interference filters.

All irradiations were performed in a specially designed cuvette, as previously described.37 Light intensities were determined by potassium tris(oxalate)ferrate(III) actinometry before and after each photolysis. Apparent trans-to-cis photoisomerization quantum yields (Φapp trans→cis) were obtained following absorbance changes by eq 1 (Tables S1−S5). The latter are “apparent” because both trans and cis isomers absorb in the same region.8,9,20−22,24,27,30

Φapp trans → cis =

1 (A trans − Airr )NAVirr I0t irr εtransb

(1) −1

In eq 1, tirr = irradiation time (s), I0 = light intensity (quanta s ), b = optical path length of the irradiated cuvette (cm), Virr = volume of the irradiated solution, and NA = Avogadro number. εcis (λ) was obtained using the ratio between the integrals of the trans and cis 1H NMR signals in an irradiated solution (eq 2) 1

εcis(λ) = εtrans(λ)

H NMR Airr (λ) − A trans (λ) %trans 1

A trans(λ) %cisH NMR

(2)

in which εtrans(λ) = molar absorptivity for the trans-isomer complex (L 1

1

H NMR mol−1 cm−1), %trans and %cisH NMR = percentages of trans- and cisisomers in the irradiated solution obtained by the distinct integrals in

C


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Inorganic Chemistry

Figure 1. 1H NMR spectra for fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in CD3CN at 298 K: (A) 200 MHz; (B) 800 MHz. the 1H NMR spectrum, Atrans(λ) = absorbance of the trans-isomer solution before irradiation, and Airr(λ) = absorbance of the irradiated solution. True quantum yields (Φtrue trans→cis) were obtained by correcting app Φtrans→cis to the molar absorptivity of the cis-isomer [εcis(λ)/L mol−1 cm−1] using eq 3 (Tables S1−S5) and just named as Φtrans→cis hereafter. Φtrue trans → cis =

1 (Airr − A trans )NAVirr I0tirr (εcis − εtrans)b

Low-temperature emission spectra were obtained in glassy (propionitrile/butyronitrile, 4:5, v/v) at 77 K using a quartz tube inside a Dewar flask filled with liquid nitrogen.

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RESULTS AND DISCUSSION

Initially, a conventional66−69 synthetic procedure for synthesis of the complex was explored by reacting the fac-[Re(CO)3(dcbH2)Cl] precursor with trans-stpy but without significant product formation. The direct reaction of transstpy coordination with the acid precursor fac-[Re(CO)3(dcbH2)Cl] posed a significant challenge because of neutralization between trans-stpy and the carboxylic acid group dcbH2. The desired product was obtained by using a strategy to protect to the acid group starting with the ester precursor fac-[Re(CO)3(dmcb)Cl] to promote smooth replacement of Cl− by the trans-stpy ligand (Scheme 1, step 1). Although an additional step is required, the methodology is advantageous for being selective, fast (a hydrolysis reaction takes place in 30 min) with a good yield (78%).

(3)

Photophysical Measurements. Emission experiments were performed at room temperature (298 K) as previously described8,21,28,30 by using an ISS photon-counting spectrofluorometer, model PC1, with a 1.000 cm optical length quartz cuvette for a fluid solution. Emission measurements of pure trans-solutions were performed using a quartz flow cell (0.5 mL min−1) in order to avoid any interferences of residual cis-photoproducts formed during the experiments. Emission quantum yields of the complexes in degassed acetonitrile were determined against fac-[Re(CO)3(bpy)Cl] (ϕ = 0.0060)64 using a procedure previously described.65 D


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Inorganic Chemistry Table 2. 1H NMR Data for fac-[Re(CO)3(dcbH2)(transstpy)]+ in CD3CN at 298 K

Figure 3. Changes in the absorption spectrum for fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in CH3CN as a function of the photolysis time, leading to a PS (λirr = 436 nm, Δt = 5 s, and T = 298 K).

Table 4. Isosbestic Points Reported for fac[Re(CO)3(R2bpy)(trans-L)]+ in Absorption Changes upon trans-to-cis Isomerization

Table 3. Spectral Parameters for fac[Re(CO)3(dcbH2)(trans-stpy)]+, fac-[Re(CO)3(dcbH2)Cl], Uncoordinated trans-stpy, and trans-Hstpy+ in Acetonitrile at 298 K fac-[Re(CO)3(dcbH2) (trans-stpy)]+ fac-[Re(CO)3(dcbH2)Cl] trans-stpy trans-Hstpy+

isosbestic point

ref

284 295 285 287 284 282 284 ∼270 280

this work 71 71 38 38 39 39 17 23

signals to lower δ and dmcb hydrogen signals to higher δ because of the magnetic anisotropy from the ring-current effect.70 As is typically observed for fac-[Re(CO)3(NN)(transL)]+ complexes, trans-stpy deshields the hydrogen atoms of dmcb, while dmcb shields the hydrogen atoms of transstpy.11,14,21,22,26,38,39 The hydrogen coupling constant between Hc and Hd (J3 ≈ 16 Hz) is characteristic of trans-olefin isomers.7,11,17,55 The hydrolysis (Scheme 1, step 2) requires careful control of the reaction conditions (solvent proportion, temperature, and reaction time; see the Experimental Section), following the CO2 release, to avoid substitution of the coordinated trans-stpy ligand. The identity of the fac-[Re(CO)3(dcbH2)(trans-stpy)]+ complex was confirmed by 1H NMR spectroscopy, in particular, a characteristic broad acidic hydrogen signal (2.38 ppm) of carboxylic acid (dcbH2) and corroborated by the complete absence of the 4.02 ppm signal of methyl ester hydrogen atoms (Figure 1A). To make sure of the identity and purity of the complex, an 800 MHz 1H NMR spectrum was also collected (Figure 1B). Identification has also been ascertained by high-resolution mass spectrometry data (Figure S10). The hydrogen signals for trans-stpy (Ha,b,c,d,e,f) have practically the same chemical shifts for both fac-[Re(CO) 3 (dcbH 2 )(trans-stpy)] + (Table 2) and fac-[Re(CO)3(dmcb)(trans-stpy)]+ (Table 1), corroborating that the hydrolysis reaction does not promote substitution at the coordinated trans-stpy ligand. The absorption band for the actual complex fac-[Re(CO)3(dcbH2)(trans-stpy)]+ lies between 200 and 270 nm

Figure 2. Electronic spectra for fac-[Re(CO)3(dcbH2)(trans-stpy)]+ (black ), fac-[Re(CO)3(dcbH2)Cl] (red ), trans-stpy (blue ), and trans-Hstpy+ (blue − −) in acetonitrile at 298 K.

compound

compound fac-[Re(CO)3(dcbH2)(trans-stpy)]+ fac-[Re(CO)3(bpy)(trans-stpy)]+ fac-[Re(CO)3(Me2bpy)(trans-stpy)]+ fac-[Re(CO)3(dmcb)(trans-stpy)]+ fac-[Re(CO)3(dmcb)(trans-stpyCN)]+ fac-[Re(CO)3(bpy)(trans-stpyCN)]+ fac-[Re(CO)3(Me2bpy)(trans-stpyCN)]+ fac-[Re(CO)3(Me2bpy)(trans-bpe)]+ fac-[Re(CO)3(bpy)(trans-bpeMe)]2+

λmax, nm (ε, ×104 L mol−1 cm−1)a 221 (3.2), 325 (3.9) sh, 330 (4.2), 410 (0.27) 211 (2.6), 244 (1.6), 310 (1.1), 332 (0.67) sh, 420 (0.31) 199 (2.4), 222 (1.3), 227 (1.3), 318 (1.6) 200 (4.3), 236 (1.2), 274 (0.6), 341 (2.7)

a

sh = shoulder.

Thus, fac-[Re(CO)3(dcbH2)(trans-stpy)]+ was prepared by hydrolysis of the ester precursor fac-[Re(CO)3(dmcb)(transstpy)]+, as shown in Scheme 1, step 2, and isolated as a PF6− salt. Each step is discussed with product characterization by 1H NMR spectroscopy as follows. Table 1 summarizes 1 H NMR data for fac-[Re(CO)3(dmcb)Cl], trans-stpy, and fac-[Re(CO)3(dmcb)(trans-stpy)]+ (Scheme 1, step 1). The coordination of transstpy to fac-[Re(CO)3(dmcb)Cl] shifts trans-stpy hydrogen E


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Inorganic Chemistry

Figure 4. 1H NMR spectra for a fac-[Re(CO)3(dcbH2)(trans-stpy)]+ (black) and PS solution (gray) in CD3CN at 298 K: (A) 200 MHz; (B) 800 MHz.

and arises from a predominantly π → π* intraligand transition in the dcbH2 ligand (Figure 2). A related 1ILdcbH2 transition is also observed for fac-[Re(CO)3(dcbH2)Cl)]. The intense band from 270 to 380 nm arises from a π → π* trans-stpy transition (1ILtrans‑stpy), which is red-shifted upon ligand coordination. A similar shift is also observed for trans-Hstpy+.19,20,22,27,29,32,41,42 The lower-energy absorption band at 400−455 nm arises from a metal-to-ligand charge-transfer transition (1MLCT; dReI → π*dcbH2), compared to the spectrum of fac-[Re(CO)3(dcbH2)Cl] (Table 3). The 1MLCTRe→dcbH2 absorption band exhibits a slight blue shift after trans-stpy coordination compared to its parent complex, fac-[Re(CO)3(dcbH2)Cl]. A similar blue shift has been previously described for fac-[Re(CO)3(bpy)(py)]+ after chloride displacement by pyridine.44 The 1MLCTRe→dcbH2 transition of the actual complex is remarkably red-shifted relative to related fac-[Re(CO)3(NN)(trans-L)]+ complexes with NN = 2,2′-bipyridine and its substituted analogues (R2bpy),3−5,11,17−19,23,32,71 as a conse-

quence of electron-withdrawing stabilization of π*dcbH2 orbitals by the carboxylic group. Irradiation of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ acetonitrile solutions at 313, 334, 404, or 436 nm leads to absorption changes as a function of the photolysis time because of transto-cis isomerization (Figure 3). Continuous irradiation results in a photostationary state (PS), where the concentrations of the cis- and trans-species remain constant. These typical changes in the absorption spectrum are observed with a characteristic clear and well-defined isosbestic point at 284 nm (Table 4) without any variation in the MLCT band. Efficient trans-to-cis photoisomerization for the actual complex also occurs at 404 and 436 nm, a region in which noncoordinated trans-stpy does not absorb, demonstrating an efficient sensitization of trans-stpy coordinated to the rhenium polypyridyl complex by intramolecular energy transfer. The 1H NMR spectrum of the PS solution (Figure 4) exhibits two doublet signals with a characteristic coupling constant of cis-olefinic hydrogen atoms (J3 = 12.0 Hz)7,11,17,55 ascribed to Hc′ (6.40 ppm) and Hd′ (6.89 ppm), consistent F


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Inorganic Chemistry Table 5. 1H NMR Data for the PS Solutiona

a

Asterisks indicate signals overlapped with trans-complex hydrogen signals.

with successful photoisomerization. These signals are shifted to higher δ compared to the trans-complex because of planarity loss around the py−CHCH−Ph group,4 which leads to a strong shielding of the cis-1H nucleus, in special Hb′, Hc′, Hd′, and He′ atoms (Table 5). It is noteworthy that all hydrogen atoms exhibit distinct δ values in comparison to the transisomer complex because of the change of the chemical environment caused by the olefinic bond twist, similar to other rhenium(I) complexes.8,11,17,20−22,24,27,30,31,33,37−39,72 The assignments were confirmed by 1H−1H correlation spectroscopy (COSY; Figure S7). The ratio of 1H NMR integrals for cis- and trans-isomers results in a 53% cis-fraction in the PS solutions, the value used to obtain molar absorptivity of the cis-isomer complex to generate the cis-complex spectrum27 (Figure 5).

Figure 5. Electronic spectra for fac-[Re(CO)3(dcbH2)(trans-stpy)]+ (red ) and fac-[Re(CO)3(dcbH2)(cis-stpy)]+ (blue − −) in acetonitrile at 298 K. εcis(λ) values for the cis isomer were determined by eq 2.

Table 6. Quantum Yields for trans-to-cis Photoisomerization of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ and fac[Re(CO)3(R2bpy)(trans-L)]+ in Acetonitrile at 298 Ka λ irradiation compound fac-[Re(CO)3(dcbH2)(trans-stpy)]+ fac-[Re(CO)3(bpy)(trans-stpy)]+ fac-[Re(CO)3(Me2bpy)(trans-stpy)]+ fac-[Re(CO)3(dmcb)(trans-stpy)]+ fac-[Re(CO)3(bpy)(trans-stpyCN)]+ fac-[Re(CO)3(Me2bpy)(trans-stpyCN)]+ fac-[Re(CO)3(dmcb)(trans-stpyCN)]+ fac-[Re(CO)3(bpy)(trans-stpyNO2)]+ fac-[Re(CO)3(bpy)(trans-stpyODA)]+ fac-[Re(CO)3(bpy)(trans-stpyODO)]+ fac-[Re(CO)3(bpy)(trans-bpe)]+ fac-[Re(CO)3(Me2bpy)(trans-bpe)]+

313 nm

334 nm

365 nm

404 nm

436 nm

ref

0.49 ± 0.06

0.51 ± 0.03

0.54 ± 0.05

± ± ± ±

0.53 ± 0.02 0.45 ± 0.02 0.48 ± 0.06 0.62 ± 0.05 0.66 0.49c 0.43c

0.51 ± 0.04 0.35−0.43b 0.48 ± 0.03 0.31 ± 0.07 0.43 ± 0.03 0.39 ± 0.03 0.36 ± 0.03 0.55 ± 0.05

0.50 ± 0.03 no absorption no absorption no absorption 0.44 ± 0.05 no absorption no absorption 0.56 ± 0.04 no absorption

this work 25 71 71 38 39 39 38 4 11 5, 11 17 17

0.54 0.44 0.43 0.65

± ± ± ±

0.04 0.02 0.02 0.05

0.55 0.44 0.47 0.64

350 nm

0.03 0.01 0.02 0.04

0.21 0.23

no absorption

a

trans-stpyODA = 4-[4′-(N-octadecylamide)styryl]pyridine; trans-stpyODO = N-(4-octadecyloxy-4′-styryl)pyridine-2-carbaldimine. bEstimated. In CH2Cl2.

c

G


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Inorganic Chemistry

(CO)3(dcbH2)(trans-stpy)]+ are listed in Table 6, along with values for other rhenium(I) bipyridyl complexes. Emission spectra of trans and PS solutions are pictured in Figure 6. Their maxima (∼574 nm) are similar for both complexes with negligible quantum yields (∼0.0002) under 378−400 nm excitation. Usually, trans-stilbene rhenium(I) complexes are practically nonemissive compared to their emissive cis-isomers.4,6,8,9,14,16,17,20−22,24,26,30,38,42 The enhanced emission of cis-complexes, as reported previously, arises from a change in the nature of the lowest-lying excited state22 and results from the destabilization of ILcis‑stpy states, while MLCTRe→NN remains constant and becomes the lowest state.9,13,22,30 In spite of the low emission of the fac-[Re(CO)3(dcbH2)(cis-stpy)]+ complex, the lowest-lying excited state is 3MLCT. This emission is lower than usual because it may be quenched by the acid group and is enhanced upon the addition of an external base (Figure S8). The identity of this 3MLCT emission is also demonstrated by a rigidochromic effect,73,74 which shows a hypsochromic shift of the emission at 77 K (Figure S9). Irradiation of the PS solution of fac-[Re(CO)3(dcbH2)(stpy)]+ at 255 nm does not lead to spectral changes arising from cis-to-trans photoisomerization, and there was no evidence for a back-reaction under these conditions (Figure 7). A schematic energy diagram to explain the photochemical and photophysical behavior is proposed in Figure 8. The photochemical behavior proposed is in accordance with the diagram proposed previously for the isomerization of coordinated trans-bpe, followed by time-resolved IR measurements that demonstrate that photoisomerization occurs through a triplet mechanism.13 It is noteworthy that the isomerization quantum yields for fac-[Re(CO)3(dcbH2)(trans-stpy)]+ do not depend on the irradiation wavelength from 313 to 436 nm, displaying an effective sensitization toward visible excitation through the 3 MLCT state. The wavelength dependence reported previously by Faustino et al.1 seems to be more appropriate for an isomerization of trans-Hstpy+. Also, it is noteworthy the similarity of the 1H NMR spectra that they show (ref 1, Figure 4, trans, p 2936) compare with the trans-Hstpy+ 1H NMR (Figure 9A) and its variation upon photoisomerization (Figure S5). One of the possible rhenium complexes observed in their 1 H NMR spectra1 is a solvento complex. Here, we synthesized fac-[Re(CO)3(dcbH2)(CD3CN)]+, and its 1H NMR spectrum is shown in Figure 9B. Figure 9C is an overlap of trans-Hstpy+ and fac-[Re(CO)3(dcbH2)(CD3CN)]+, which is very similar to Figure 4, trans, of the previous work. Moreover, there are other inconsistences in their data, such as MLCT band variations and the isosbestic point, that are not observed for other fac-[Re(R2bpy)(trans-L)]+ complexes and 1 H NMR assignments. For the latter, they report 1H NMR spectral changes that are appropriate for the behavior of the noncoordinated ligand with the absence of NN (Hα, Hβ, and Hγ) signal changes that appear for cis-complexes. In all literature data, there are clearly distinct NN signals for both trans- and cis-complexes.11,17,20−22,24,27,28,30,31,33,37−39,71,72

Figure 6. Emission spectra for fac-[Re(CO)3(dcbH2)(trans-stpy)]+ (black ), PS acetonitrile solution (gray ), and standard fac[Re(CO)3(bpy)Cl)] (pink - - -). The inset shows amplified emission spectra (λexc = 378 nm, 298 K) for the trans complex and PS solution (λirr = 436 nm and tirr = 10 min).

Figure 7. Electronic absorption spectra of a PS solution with 255 nm irradiation at 298 K. The inset shows absorption at 330 nm versus irradiation time.

Figure 8. Simplified energy diagram.

The intraligand π → π* transition for stpy, 1ILstpy, occurs mainly in the 270−380 nm range. The 1ILcis‑stpy transition is blue-shifted compared to 1ILtrans‑stpy because of the planarity loss and a consequent π* destabilization. The Φtrans→cis values following irradiation at 313, 334, 365, 404, or 436 nm are constant and independent of the excitation energy within experimental error, confirming the same reaction pathway with population of a lowest-lying 3ILstpy excited state and a wavelength-independent mechanism, previously reported only for the bpe complexes.42 The average quantum yields, Φtrans→cis, for trans-to-cis photoisomerization of fac-[Re-

■

CONCLUSION The data reported in this paper are inconsistent with the conclusions reached in a previously published paper.1 Here we H


Article

Inorganic Chemistry

Figure 9. 1H NMR spectra (300 MHz) in CD3CN at 298 K for (A) trans-Hstpy+, (B) fac-[Re(CO)3(dcbH2)(CD3CN)]+, and (C) an overlap of A and B.

■

report a step-by-step synthetic procedure for the proposed fac[Re(CO)3(dcbH2)(trans-stpy)]+ with 1H NMR assignments at each step. We also demonstrate behavior expected for the actual complex with effective photosensitization up to 436 nm and an important possible role as a 3MLCT sensitizer for potential applications in solar energy conversion. The distinction is important because of the promise of the chromophore for applications in solar-light-induced devices.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lais S. Matos: 0000-0001-9119-3984 Ronaldo C. Amaral: 0000-0002-1729-3511 Neyde Y. Murakami Iha: 0000-0002-3262-6484 Author Contributions

ASSOCIATED CONTENT

All authors have given approval to the final version of the manuscript.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01304.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by Fundaçaõ de Amparo à Pesquisa do Estado de São Paulo, Coordenaçaõ de Aperfeiçoamento de Pessoal de Nı ́vel Superior, and Conselho Nacional de Desenvolvimento Cientı ́fico e Tecnológico. The authors are thankful to Gizele Celante, Marcos Archilha, and Alcindo dos Santos for 200 MHz 1H NMR spectra.

1

H NMR spectra for trans-stpy, trans-Hstpy+ and its irradiated solutions, fac-[Re(CO)3(dmcb)Cl]+, fac-[Re(CO)3(dmcb)(stpy)]+, and the PS solution for fac[Re(CO)3(dcbH2)(stpy)]+, COSY NMR for the PS solution for fac-[Re(CO)3(dcbH2)(stpy)]+, emission spectral changes for fac-[Re(CO)3(dcbH2)(stpy)]+ as dcbH2 is deprotonated, emission spectra for fac-[Re(CO)3(dcbH2)(stpy)]+ in propionitrile/butyronitrile (4:5, v/v) at 77 K, and complete triplicate data for the quantum yield calculations (PDF)

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REFERENCES

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Visible Photosensitization of trans-Styrylpyridine Coordinated to fac

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