Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Photochemistry of fac-[Re(CO)3(dcbH2)(trans-stpy)]+: New Insights on the Isomerization Mechanism of Coordinated Stilbene-like Ligands Leandro A. Faustino, Antonio Eduardo Hora Machado, and Antonio Otavio T. Patrocinio* Laboratory of Photochemistry and Materials Science, Chemistry Institute, Federal University of Uberlândia, Uberlândia 38400-902, Minas Gerais, Brazil S Supporting Information *
ABSTRACT: In this work, a novel complex fac-[Re(CO)3(dcbH2)(trans-stpy)]+, (dcbH2 = 4,4′-dicarboxylic acid-2,2′-bipyridine; trans-stpy = trans-4-styrylpyridine) was synthesized and characterized toward its spectroscopic, photochemical, and photophysical properties. The experimental data provide new insights on the mechanism of photochemical trans-to-cis isomerization of the stilbene-like ligand coordinated to Re(I) polypyridyl complexes. The new complex exhibits an unusual and strong dependence of the isomerization quantum yield (Φt→c) on the irradiation wavelength. Φt→c was 0.81 ± 0.08 for irradiation at 365 nm and continuously decreased as the irradiation wavelength is shifted to the visible. At 405 nm irradiation Φt→c is almost 2 orders of magnitude lower (0.010 ± 0.005) than that observed at 365 nm excitation. This behavior can be explained by the low-lying triplet metal-to-ligand charge-transfer excited state (3MLCT) that hinders the triplet photoreaction mechanism under visible light absorption. Under UV irradiation, direct population of styrylpyridine-centered excited state (1IL) leads to the occurrence of the photoisomerization via a singlet mechanism. Further experiments were performed with the complex immobilized on the surface of TiO2 and Al2O3 films. The nonoccurrence of isomerization at the oxide surfaces even under UV excitation evidences the role of energy gap between the 1IL/1MLCT states on the photochemical/photophysical processes. The results establish important relationships between the molecular structure and the photoelectrochemical behavior, which can further contribute to the development of solid-state molecular switches based on Re(I) polypyridyl complexes.
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INTRODUCTION The photochemistry of Re(I) polypyridyl complexes having an isomerizable stilbene-like ancillary ligand have attracted great attention in the last years due to the possibility to developing photoinduced molecular switches and sensors. Since the first reports by Wrighton and co-workers,1 a growing series of fac[Re(CO)3(NN)(trans-L)]+ complexes (NN = polypyridyl ligand, trans-L = stilbene-like ligand) have been reported.2−17 Complexation of the isomerizable ligand may trigger the photoreaction in spectral regions in which the ligand itself does not absorb. As shown by time-resolved techniques,18,19 the reaction mechanism involves an intramolecular energy transfer at picosecond scale from the triplet metal-to-ligand charge transfer (3MLCTRe→NN) excited state to the respective ligandcentered state (3ILtrans‑L). Different reaction pathways, however, may take place depending on the choice of the polypyridyl and the isomerizable ligands. Particularly, for the trans-4-styrylpyridine (trans-stpy) ligand, we have shown a dependence of the isomerization quantum yield on the irradiation wavelength, which was speculated to be due the occurrence of a possible singlet mechanism under UV excitation.13 Theoretical studies performed by Daniel et al. using state-of-the-art ab initio methods20 have shown that the dynamics of the photoisomerization process is dependent on the energy levels of 1,3 MLCT and 1,3IL excited states and that both, singlet and triplet, mechanisms can occur. However, few experimental data © XXXX American Chemical Society
are reported on the relative contribution of each reaction pathway as a function of the irradiation wavelength. In this work, a novel complex fac-[Re(CO)3(dcbH2)(transstpy)]+, (dcbH2 = 4,4′-dicarboxylic acid-2,2′-bipyridine) was synthesized, and its photochemical properties were investigated in detail. In this complex, the isomerization quantum yield exhibits strong wavelength dependence, much higher than those reported before. This behavior is rationalized in terms of the energy levels of the 1,3MLCT and 1,3IL excited states providing experimental evidence for the photoreaction via a singlet mechanism.
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EXPERIMENTAL SECTION
Materials. All chemicals were reagent grade and were used without purification. Benzaldehyde, 4-picoline, HPF6, 4,4′-dicarboxylic acid2,2′-bipyridine (dcbH2), silver trifluoromethane sultanate (AgTFMS), aluminum isopropoxide, [ClRe(CO3)5], lithium iodide, and Carbowax 20 M were acquired from Aldrich. The ligand trans-stpy was synthesized as previously reported.21 Potassium tris(oxalate)ferrate(III) employed as chemical actinometer was prepared and purified according to the literature procedure.22 fac-[ClRe(CO)3(dcbH2)] was also prepared as reported elsewhere.23 All solvents employed on the photoelectrochemical experiments were high-performance liquid chromatography (HPLC) grade. Received: January 12, 2018
A
DOI: 10.1021/acs.inorgchem.8b00093 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Syntheses. fac-[Re(CO)3(dcbH2)(trans-stpy)]PF6. The complex was synthesized by slight modifications of the reported procedures for similar complexes.24−26 0.24 g (0.43 mmol) of [ClRe(CO)3(dcbH2)] was suspended in 35 mL of tetrahydrofuran (THF), and 0.11 g (0.43 mmol) of AgTFMS was added to the mixture under argon atmosphere. After 2 h of continuous stirring, the formed AgCl was removed by filtration. 0.79 g (4.38 mmol) of trans-stpy previously solubilized in THF was added to the filtrate, and the mixture was refluxed for 6 h. After it cooled, a red solid was formed and separated by filtration. The crude product was recrystallized by dissolution in ethanol and slow addition of diluted HPF6. Yield 0.11 g (30%). 1H RMN (CD3CN δ/ppm) 9.21 (d, 2H); 8.95 (s, 2H); 8.56 (d, 2H); 8.09 (dd, 2H); 8.05 (d, 2H); 7.85 (d,1H); 7.75 (dd, 2H); 7.51 (m, 3H); 7.40 (d, 1H). Anal. Calcd for ReC28H19N3O7PF6: C, 40.01%; H, 2.28%; N, 5.00%; Found: C, 39.86%; H, 1.98%; N, 4.98%. Sensitization of Metal Oxide Films. TiO2 films were deposited over transparent fluorine-doped tin oxide (FTO) substrates (TEC15Pilkinton Co.) by screen printing using a commercial TiO2 paste (DSL 18 NR-T, Dye-Sol). After they dried at room temperature, the electrodes were sintered at 450 °C for 30 min. Before deposition of the Al2O3 thin films, the oxide nanoparticles were prepared by the sol−gel technique. 8.1 g (40 mmol) of aluminum isopropoxide was solubilized in 60 mL of ethanol, and, under vigorous stirring, 75 mL of aqueous 0.1 mol L−1 HNO3 solution was added dropwise. After it aged at 80 °C for 12 h, the sol was submitted to a hydrothermal treatment at 200 °C and 150 psi for 8 h to yield spherical Al2O3 nanoparticles. The powder was then suspended in water and stabilized by the addition of Carbowax 20 M (40% m/m) to form a paste that was used to film deposition on FTO by doctor blading. The resulting films were then sintered at 450 °C for 30 min. Sensitization of the TiO2 or Al2O3 surfaces was achieved by soaking the sintered electrodes in 1 × 10−5 mol L−1 acetonitrile solutions of fac-[Re(CO)3(dcbH2)(trans-stpy)]PF6. Methods. Electronic absorption spectra were recorded on a double beam Shimadzu UV-1650 spectrophotometer. 1H NMR spectra were recorded on a DRX-400 MHz Bruker Ascend 400 spectrometer, and the residual solvent signals were used as internal standard. Attenuated total reflectance infrared (ATR-FTIR) spectra were recorded in a PerkinElmer Frontier spectrometer. The measurements were recorded in a diamond crystal plate, using 16 scans at a resolution of 4 cm−1. For the metal oxide films, the bare oxides were used as background. Photolysis experiments were performed using a Newport 300 W Xe lamp connected to a monochromator (Newport 7415) to wavelength selection. 3 mm slits were employed in all experiments ensuring a spectral resolution of ±4.5 nm. A WG320 long-pass filter was positioned before the sample to avoid UVB or UVC photons due to second (or higher) order diffraction in the monochromator grating. Light intensities were determined by tris(oxalate)ferrate(III) actinometry before and after each photolysis. The solutions were irradiated in a closed 1.000 cm quartz cuvette under stirring and monitored spectrophotometrically as a function of the irradiation time. For the sensitized films, the photolysis assays were performed in air with the films parallel to the cuvette walls. A mask was used to limit the irradiated area. Photocurrent action spectra were collected in a photoelectrochemical cell, assembled in a homemade Teflon adapter. A Pt-covered FTO was employed as counterelectrode, and a 0.5 mol L−1 LiI solution in acetonitrile was used as electrolyte. The adsorbedphoton-to-current conversion efficiencies (APCE) values were calculated using eq 1,27 in which Jph is the photcurrent density, λ is the irradiation wavelength, P0 is the photon flux in mW cm−2, and A is the absorbance at the irradiation wavelength. The light intensity was measured with a Newport 818-UV calibrated photodiode connected to a 1916-R power meter. The photocurrent was recorded in a NOVA potentiostat/galvanostat (Autolab).
APCE =
measurements are underestimated, since the photoproduct absorbs in the same spectral region as the reagent. Thus, 1H NMR spectroscopy was used to determine the relative concentration of the trans and cis species at photostationary solution (PSS), which was then combined with the spectrophotometric data to obtain the molar absorptivity of the photoproduct (εcis), eq 2 (Figure S1). In the equation, A is the absorbance of irradiated solution, εtrans is the trans isomer molar absorptivity, and b is the optical path. Once the molar absorptivity of the cis-complex isomer is determined, the molar concentration of the photoproduct in all irradiated solutions can be accurately determined using eq 2. The isomerization quantum yields (Φt→c) were then obtained from eq 3, in which nr is the number of species that undergoes the photoreaction, I0 is the incident light intensity, At is the absorbance of the solution at the irradiation wavelength at a given photolysis time, and t is the irradiation time. For the quantum yield determination, the photolysis extent was kept below 10%, and the presented results are averages of, at least, three independent experiments. Detailed information on the quantum yield determination using 1H NMR can be found elsewhere.13,28
εcis =
Φt → c =
(2)
nrt t
I0(1 − 10−A )t
(3)
Emission spectra were obtained in a FluoroMax fluorimeter (Horiba) in deaerated acetonitrile at 298 K or in 4:1 ethanol/ methanol at 77 K. The emission quantum yields were determined using [Ru(bpy)3]Cl2 as standard (ϕem = 0.062 em acetonitrile, 298 K).29 Theoretical calculations were performed using the density functional theory (DFT) as described previously.30 The electronic spectra were calculated considering the first 60 excited states using the m06 functional31 along with the Def2-TZVPP atomic basis to describe the elements (with effective core potential to describe Re).32 For calculations involving the metal oxide immobilized complex, the anatase cluster described by Jono and co-workers was employed.33 The cluster structure was kept frozen during these calculations that also involved the combination of the m06 functional with the Def2-TZVPP atomic basis mentioned above. All calculations were performed using the Gaussian09 revision E.01 software package considering the species solvated in acetonitrile. The model employed for solvent correction was the Integral Formalism of the Polarizable Continuum Model (IEFPCM),34 and it was used without symmetry restrictions. The molecular orbitals were built using the GaussView 5.0.8 software.
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RESULTS AND DISCUSSION The new complex fac-[Re(CO)3(dcbH2)(trans-stpy)]+ exhibits typical UV−vis absorption features of Re(I) polypyridyl complexes with stilbene-like ligands.10 As shown in Figure 1, at higher wavelengths, light absorption occurs through fully allowed ππ* intraligand transitions (IL) from both dcbH2 and trans-stpy ligands. Particularly, for the ππ*trans‑stpy, a bathochromic shift from 305 to 345 nm is observed after coordination, similarly to that observed after protonation. Above 390 nm, a broad absorption band is observed with molar absorptivities in the order of 1 × 103 L mol−1 cm−1 and attributed to the metal-to-ligand charge transfer from the Re(I) center to the dcbH2 ligand (MLCTRe→dcbH2). As shown in Figure 1, the change of the ancillary ligand from Cl− to transstpy does not significantly shift the MLCTRe→dcbH2 absorption maximum. As expected due to the presence of electron withdrawing carboxylate groups, this band is ca. 40 nm redshifted in relation to that for the parental complex fac[Re(CO)3(bpy)(trans-stpy)]+, bpy = 2,2′-bypyridine.18 Timedependent (TD) DFT calculations allow a detailed description
(1240 eV· nm)Jph λP0(1 − 10−A)
A − εtrans· b· C trans Ccis·b
(1)
13,14
the trans-to-cis photoisomerization As discussed before, quantum yields of Re(I) complexes determined only by UV−vis B
DOI: 10.1021/acs.inorgchem.8b00093 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Electronic spectra of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ (black), free (blue) and protonated (blue ---) trans-stpy, and fac[ClRe(CO)3(dcbH2)] (red) in acetonitrile. The vertical lines correspond to the calculated TD-DFT electronic transitions of fac[Re(CO)3(dcbH2)(trans-stpy)]+.
Figure 2. Isosurface plots of frontier orbitals of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in acetonitrile.
of the molecular orbitals involved in the electronic transitions of the complex, Table 1. Isosurface plots of the frontier orbitals are shown in Figure 2.
nm, which will directly affect the photochemical properties of the complex as described below. Irradiation of the fac-[Re(CO)3(dcbH2)(trans-stpy)]+ complex in acetonitrile at 365 nm leads to spectral changes with an isosbestic point at 378 nm, Figure 3. These changes are
Table 1. Solvent-Corrected (Acetonitrile) Major Transition Energies to the Excited States of fac[Re(CO)3(dcbH2)(trans-stpy)]+ with Their Contributing Excitations (%), Oscillator Strengths (f), and Associated Wavelengths (λ) f
energy (eV)
λ (nm)
character
H−L (75%)
0.0074
2.55
484
H-2−L (62%) H-3−L (63%) H−L+1 (82%) H−L+2 (18%)
0.0246 0.1190 1.0649
2.86 2.99 3.39
433 414 365
H-1−L+1 (58%)
0.1172
3.88
319
H-5−L (71%) H-3−L+1 (12%)
0.3647
3.99
310
H-5−L+3 (82%) H-7−L (12%)
0.1662
MLCTd(Re)→π*dcbH2 LLCTπstpy→π*dcbH2 MLCTd(Re)→π*dcbH2 MLCTd(Re)→π*dcbH2 MLCTd(Re)→π*dcbH2 LLCTπstpy→π*dcbH2 MLCTd(Re)→π*stpy ILπstpy→π*stpy MLCTd(Re)→π*dcbH2 LLCTπstpy→π*dcbH2 MLCTd(Re)→π*stpy ILπstpy→π*stpy MLCTd(Re)→π*dcbH2 ILπdcbH2→π*dcbH2 MLCTd(Re)→π*stpy ILπdcbH2→π*dcbH2 LLCTπstpy→π*dcbH2
transitiona
a
4.95
250
Figure 3. Spectral changes of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in acetonitrile under 356 nm irradiation (Δt = 60 s); I0 = 2.5 × 1014 quanta s−1.
H = HOMO, L = LUMO.
attributed to the trans-to-cis isomerization of the coordinated stpy ligand, as previously described for similar complexes. 1H NMR spectroscopy was employed to confirm the formation of the photoproduct fac-[Re(CO)3(dcbH2)(cis-stpy)]+, Figure 4. While olefin protons in the coordinated trans-stpy at 7.85 and 7.40 ppm exhibit a coupling constant (J) of 16.4 Hz, the same protons in the coordinate cis isomer has J = 12.5 Hz, Table 2. These values agree with those typically reported for coordinated trans and cis stilbenes28 and confirm the formation of the expected photoproduct. The proton assignments were also corroborated by H−H correlation spectroscopy (COSY) spectra (Figure S2, Supporting Information). 1H NMR data can also be used to determine the ratio between the trans and cis
The calculations corroborate with the experimental attributions, evidencing that the lowest unoccupied molecular orbital (LUMO) in the complex is centered in the π* orbitals of the dcbH2 ligand, while the highest energy occupied orbital (HOMO) is composed by d orbitals from the metal center and π orbitals from trans-stpy. Thus, the lowest-lying absorption bands are in fact MLCT/LLCT (LLCT = ligandto-ligand charge transfer) in character. It is noteworthy to observe that the population of the π* of the trans-stpy ligand (LUMO+1/LUMO+2) occurs at wavelengths longer than 400 C
DOI: 10.1021/acs.inorgchem.8b00093 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
species in the PSS, which for fac-[Re(CO)3(dcbH2)(stpy)]+ in acetonitrile was 18:82; that is, 82% of the trans species is converted into its cis counterpart. From these data, it is possible to determine the molar absorptivity of the photoproduct, fac[Re(CO)3(dcbH2)(cis-stpy)]+ and then the accurate quantum yield for the photoisomerization reaction. If the complex solution is irradiated with longer wavelengths, such as 405 nm, the formation of the photoproduct can also be observed (Figure S3, Supporting Information); however, the quantum yield of the photoreaction was strongly affected by the irradiation wavelength. The determined values are listed in Table 3, along with data for other Re(I) complexes. The complex fac-[Re(CO)3(dcbH2)(trans-stpy)]+ exhibits a distinct behavior in relation to other Re(I) complexes reported. In the complex with the dcbH2 ligand, irradiation at 365 nm leads to efficient trans-to-cis isomerization of stpy with quantum efficiency higher than that for the protonated ligand under same conditions. As the irradiation wavelength is shifted to 405 nm, the photoinduced trans-to-cis isomerization process is almost fully suppressed, and the observed quantum yield is 2 orders of magnitude lower than that at 365 nm. For the other Re(I) polypyridyl complexes with the trans-stpy ligand, a decrease of ca. 30% in the quantum yield is observed as the irradiation wavelength is changed from 365 to 405 nm. Such a behavior is not observed for complexes with the trans-bpe ligand, in which the quantum yield is independent of the excitation wavelength.10,20 To possibly explain the distinct photochemical behavior observed for fac-[Re(CO)3(dcbH2)(trans-stpy)]+, it is necessary to look at the possible photoisomerization mechanisms of coordinated stilbene-like (L) ligands in Re(I) polypyridyl complexes. First experimental evidence for the so-called photosensitized isomerization of stilbene-like ligands in Re(I) complexes were given by Iha and Meyer19 by means of timeresolved infrared spectroscopy measurements using the fac[Re(CO)3(phen)(trans-bpe)]+ complex. The authors showed that the photochemical reaction proceeds through a triplet mechanism: excitation of 1MLCTRe→phen or 1ILtrans‑bpe is followed by intersystem crossing for the 3MLCTRe→phen excited state, which then sensitizes the 3ILtrans‑bpe, responsible for the photoreaction. The isomerization occurs by the formation of an intermediate state (3ILper), in which the aromatic rings in the stilbene ligand are perpendicular to each other. Later, Vlcek and co-workers used time-resolved infrared and Raman spectroscopies to investigate the behavior of fac-[Re(CO)3(bpy)(trans-
Figure 4. 1H NMR (400 MHz) spectral changes of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in CD3CN under 356 nm irradiation.
Table 2. 1H NMR Spectral Attributions for fac[Re(CO)3(dcbH2)(stpy)]+ Complexes
Table 3. Quantum Yields for the trans-to-cis Photoisomerization of Different Compounds in Acetonitrile at 365 and 405 nm Excitation compounda
Φ(365 nm)
Φ (405 nm)
ref
protonated trans-stpy fac-[Re(CO)3(dcbH2)(trans-stpy)]+ fac-[Re(CO)3(bpy)(trans-stpy)]+ fac-[Re(CO)3(phen)(trans-stpy)]+ fac-[Re(CO)3(ph2phen)(trans-stpy)]+ fac-[Re(CO)3(Me4phen)(trans-stpy)]+ fac-[Re(CO)3(phen)(trans-bpe)]+ fac-[Re(CO)3(ph2phen)(trans-bpe)]+ fac-[Re(CO)3(Me4phen)(trans-bpe)]+
0.58 ± 0.04 0.81 ± 0.08
no absorption 0.010 ± 0.005 0.48 ± 0.03 0.43 ± 0.02 0.42 ± 0.03 0.35 ± 0.02 0.77 ± 0.09 0.43 ± 0.02 0.29 ± 0.03
35 this work 36 28 28 13 28 28 13
0.60 0.64 0.57 0.80 0.44 0.33
± ± ± ± ± ±
0.06 0.09 0.06 0.07 0.02 0.04
a
phen = 1,10-phenanthroline; ph2phen = 4,7-diphenyl-1,10-phenanthroline; Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline; bpe = 1,2-bis(4pyridylethylene). D
DOI: 10.1021/acs.inorgchem.8b00093 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry stpy)]+ and fac-[ClRe(CO)3(trans-stpy)2]+, and a similar mechanism was proposed.18 Further insights on the role of the excitation wavelength on the photoisomerization mechanism were given by Daniel et al. by means of ab initio calculations.20,37−40 The authors concluded that three different reaction pathways may occur depending on the excitation wavelength and the energy gap between the 1ILtrans‑L and 1,3MLCTRe→NN states, Figure 5. The
Figure 6. Emission spectra of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ (―), photostationary solution with 82% of fac-[Re(CO)3(dcbH2)(cis-stpy)]+ (red ―) in acetonitrile at 298 K; λexc = 405 nm.
the cis isomer are luminescent. This behavior is explained by the destabilization of ILtrans‑stpy states after the photoreaction, while the emissive MLCTRe→NN states remain unchanged. In rigid medium (ethanol/methanol 4:1 at 77 K), it is also possible to observe the so-called rigidochromic effect for fac[Re(CO)3(dcbH2)(trans-stpy)]+ with a blue shift in the emission maximum of ca. 30 nm in relation to that at room temperature (Figure S4, Supporting Information). This behavior is typical of MLCT emitters. The energies of the first triplet states for both trans and cis species were calculated by TD-DFT (Figure S5) along with some natural transition orbitals (NTOs) for the low-energy transitions (Figure S6). For the fac-[Re(CO)3(dcbH2)(trans-stpy)]+, the calculation predicts that the first Franck−Condon triplet state is actually 3 ILtrans‑stpy in character, lying ca. 0.2 eV below the second triplet state, which is MLCT. After isomerization, the 3IL is destabilized by ca. 0.3 eV, and in fac-[Re(CO)3(dcbH2)(cisstpy)]+ the lowest triplet state is MLCT in character with similar energy as that for the trans species. The predicted energies for the lowest MLCT and IL triplet states in both complexes are very close, lying into the expected deviation for the methodology applied in this study. On the basis of the experimental data, it seems to be reasonable to conclude that in both fac-[Re(CO) 3 (dcbH 2 )(trans-stpy)] + and fac-[Re(CO)3(dcbH2)(cis-stpy)]+, the 3MLCTRe→dcbH2 state is the low-lying triplet excited state after thermalization. In contrast to the low photoisomerization quantum yield observed under visible light excitation, for 365 nm irradiation, the process occurs very efficiently, which means that, at this wavelength, the singlet pathway through routes 2 or 3 are activated. It’s noteworthy to observe that the determined quantum yield at 365 nm is actually 10−15% higher than those reported for other Re(I) complexes, which may be related to the relatively stronger contribution of the LLCTtrans‑stpy→dcbH2, states for the electronic transitions that likely reduces the activation energy associated with the CC torsion in the olefin ligand. Experimental distinction between routes 2 and 3 is challenging. By probing the trans-to-cis isomerization quantum yields at irradiation wavelengths between 365 and 405 nm, Figure 7, it could be observed that the values decrease almost linearly between 365 and 395 nm, reaching a minimum at 405 nm. It is noteworthy that the absorption edge for the protonated trans-stpy, in which the same spectral shift of the
Figure 5. Possible mechanisms for the trans-to-cis isomerization in fac[Re(I)(CO)3(NN)(trans-L)]+ complexes as proposed by Daniel et al. Adapted from refs 20 and 40.
route 1 is triggered under visible light excitation and involves the population of 1,3MLCTRe→NN followed by an intramolecular energy transfer to 3ILtrans‑L. The routes 2 and 3 can be activated at higher excitation energies and will lead to the isomerization by a singlet pathway, that is, through the population of the 1 ILtrans‑L state. The route 2 will depend on the location of the conical intersection between the 1MLCT and the 1IL states, whereas the route 3 can be triggered by direct excitation of IL(ππ*) transitions centered at the coordinated stilbene-like ligand. On the one hand, the nondependence of the isomerization quantum yield for the complexes with the trans-bpe ligand suggests that the route 1 is the only reaction pathway in this case. On the order hand, for the complexes with trans-stpy the routes 2 or 3 seem to be activated at high excitation energy, in which the quantum yields are higher. On the basis of the mechanisms proposed by Daniel et al., we can then look at the photochemical behavior of the fac[Re(CO)3(dcbH2)(trans-stpy)]+. The reduced photoinduced isomerization quantum yield at 405 nm excitation indicates that the route 1 is almost completely quenched. This behavior can be attributed to the stabilization of the charge-transfer states due to the introduction of electron-withdrawing carboxylate groups in the bpy ligand. In fact, TD-DFT calculations show that the low-lying singlet states are MLCT Re→dcbH2 / LLCTstpy→dcbH2, and, additionally, the trans isomer exhibits luminescence at room temperature with the same quantum yield that the chloro complex fac-[ClRe(CO)3(dcbH2)] (ϕ ≈ 1 × 10−3). The emission band of fac-[Re(CO)3(dcbH2)(transstpy)]+ is broad and poorly structured with maximum at 600 nm, Figure 6. Moreover, its emission profile is similar to that observed for the photostationary solution, with 82% of fac[Re(CO)3(dcbH2)(cis-stpy)]+. The emission band also can be observed with other excitation energies, such as 365 nm and with similar quantum yield. Most of Re(I) polypyridyl complexes with trans-stpy are nonemissive at room temperature, while the complexes with E
DOI: 10.1021/acs.inorgchem.8b00093 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 8. Electronic spectra of bare TiO2 (black ---), bare Al2O3(blue ---), TiO2 (black line), and Al2O3 (blue line) films sensitized with fac[Re(CO)3(dcbH2)(trans-stpy)]+ and TiO2 film sensitized by the PS solution (red line).
Figure 7. Wavelength dependence of trans-to-cis isomerization quantum yield of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in acetonitrile at 298 K. 1
ILtrans‑stpy observed upon coordination occurs, is around 400 nm. It is therefore reasonable to consider that direct population of the 1ILtrans‑stpy can still occur at 395 nm, competing with the MLCT transition. As the irradiation wavelength is changed from 365 to 395 nm, the contribution of 1ILtrans‑stpy for the light absorption decreases, and concomitantly the isomerization quantum yield also decreases 1 order of magnitude. This behavior suggests that the route 3 is the major isomerization pathway, despite the contribution of route 2 cannot be fully excluded. In one attempt to have insights about the major photoisomerization pathway, the complex was adsorbed on the surface of metal oxides through the carboxylate groups. The immobilization of photoisomerizable complexes in rigid matrices is an important step toward developing molecular photoswitches. For the Re(I) complex studied here, the immobilization should lead to the stabilization of the MLCT state, while the ILtrans‑stpy will remain relatively unchanged. Two oxides were selected: TiO2 anatase (Eg = 3.2 eV), in which electron injection is thermodynamically favored, and Al2O3, in which the conduction band potential is much more negative, and the electron injection by the photoexcited Re(I) species is uphill. The disappearance of the FTIR absorption bands related to the stretching of carboxylate group in the dcbH2 ligand after adsorption confirms that the complex was covalently bound to the oxide surfaces (Figure S7, Supporting Information). In Figure 8, in the absorption spectra of the sensitized films, one can observe a broadening of the lowest-lying absorption band on the oxide surfaces as a result of the stabilization of the MLCTRe→dcbH2 excited state. Excitation of the TiO2 film sensitized with fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in air does not lead to any measurable spectral changes when a steady-state UV−vis spectrometer is used. In fact, under excitation, fac-[Re(CO)3(dcbH2)(trans-stpy)]+ is able to inject electrons on TiO2 conduction band yielding an anodic current, Figure 9. For comparison, a TiO2 film was also sensitized with photostationary solution having 82% of the photoproduct (electronic spectrum in Figure 8) and the APCE of both isomers were compared, Figure 10. As TiO2 is a stronger UVA absorber, the photocurrent obtained for the bare film was first subtracted from those measured for the sensitized ones, so the photoresponse attributed to only the Re(I) complexes could be evaluated.
Figure 9. Photocurrent response of TiO2 films sensitized by fac[Re(CO)3(dcbH2)(trans-stpy)]+ in 0.5 LiI mol−1 L acetonitrile solution under 365 nm (blue) and 405 nm (red) excitation.
Figure 10. APCE spectra of TiO2 thin film sensitized by fac[Re(CO)3(dcbH2)(trans-stpy)]+ (black) and of a PS solution having 82% of fac-[Re(CO)3(dcbH2)(cis-stpy)]+ (red).
Evaluation of the trans-to-cis isomerization on the film surface is difficult, since TiO2 strongly absorbs light up to 390 nm. APCE measurements reveal that the cis isomer injects electrons more efficiently than the trans isomer, and therefore an increase in the photocurrent should be observed in the case of occurrence of the photoreaction in the TiO2 surface. As shown in Figure 9, the photocurrent remains stable for long F
DOI: 10.1021/acs.inorgchem.8b00093 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
bolt arrows, whereas secondary pathways are indicated with dashed ones.
periods, which is also indicative that the trans-to-cis isomerization of the coordinated stpy ligand is quenched on TiO2, even under 365 nm excitation. A similar behavior was observed for the sensitized Al2O3 film in which there is no electron injection, that is, no trans-to-cis isomerization was detected even with irradiation at 365 nm. The absence of isomerization on the metal oxide surfaces along with the relatively high APCE values in the UV region suggest that the π* orbitals of trans-stpy are not efficiently populated in the immobilized complex in relation to that in acetonitrile. TD-DFT calculations of the complex adsorbed in an anatase cluster reveals that the LLCTtrans‑stpy→dcbH2 transition is favored in the immobilized complex, Figure 11, leading to the
Figure 12. Proposed mechanism for the trans-to-cis isomerization of the 4-styrylpyridine ligand in the fac-[Re(CO)3(dcbH2)(trans-stpy)]+ complex.
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CONCLUSIONS
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ASSOCIATED CONTENT
The complex fac-[Re(CO)3(dcbH2)(trans-stpy)]+ was successfully synthesized and characterized. Their photochemical properties were fully investigated in fluid solution revealing a strong dependence of the isomerization quantum yield on the irradiation wavelength. Quantum yields up to 81% are observed at 365 nm, in which there is a major contribution 1IL(ππ*)trans‑stpy to the light absorption. This value decreases almost two orders of magnitude (1.0%) when the solution is irradiated at 405 nm. The observation of luminescence in fluid solution as well as TD-DFT calculations confirm that the lowest-lying excited state in the complex is MLCT/LLCT in character. Moreover, the complex is able to inject electrons into the TiO2 conduction band, as typically observed for d6 metal complexes with low-lying charge-transfer states. The experimental data suggest that the trans-to-cis isomerization in fac[Re(CO)3(dcbH2)(trans-stpy)]+ occurs mainly through a singlet mechanism, making it a unique example among other Re(I) complexes reported. The new findings can contribute to the better understanding of the photoisomerization mechanism of coordinated stilbene-like ligands in Re(I) complexes and, therefore, to their application on optomechanical molecular devices.
Figure 11. Isosurface plots of the frontier orbitals of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ adsorbed on an anatase cluster.
concentration of the electron density in the dcbH2 ligand. The first three lowest-lying unoccupied molecular orbitals are centered in the dcbH2/cluster orbitals, and the first population of π*trans‑stpy occurs at LUMO+4, ca. 1.0 eV higher in energy than LUMO. The calculated molecular orbitals and the respective absorption spectrum for the complex adsorbed on the anatase cluster can be found in the Supporting Information (Table S1, Figure S8) along with NTOs for the most important transitions (Figure S9). As evidenced by the photocurrent measurements on TiO2 surface, the occurrence of MLCT/LLCT transition leads to electron injection to the oxide conduction band. It was shown before that this process occurs within less than 100 fs from nonrelaxed MLCT states.41 The electron injection does not occur in the Al2O3 surface, where the isomerization is also not efficient. This behavior suggests that pathway 2 is not favored in the immobilized complex and, therefore, corroborates with the conclusion that pathway 3 is likely to be the main photoisomerization pathway in fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in solution under UV irradiation. On the basis of the experimental data, the possible photoisomerization mechanism for fac-[Re(CO)3(dcbH2)(trans-stpy)]+ can be illustrated as shown in Figure 12. In this scheme, the main deactivation pathways for each excited state is indicated with continuous
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00093. NMR COSY spectrum for the complex molar absorptivities for the photoproduct, 1H NMR spectral changes under 405 nm irradiation, emission spectra of fac-[Re(CO)3(dcbH2)(trans-stpy)]+ in ehanol/methanol (4:1) at 77 K and in acetonitrile, ATR-FTIR spectra of the sensitized TiO2 film, and theoretical electronic transitions for the complex adsorbed to an anatase cluster (PDF) G
DOI: 10.1021/acs.inorgchem.8b00093 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Antonio Otavio T. Patrocinio: 0000-0003-3141-3214 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Fundaçaõ de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG, PPM-00220-17, CEXAPQ-00583-13 and CEX-APQ-03017-16), Conselho Nacional ́ de Desenvolvimento Cientifico e Tecnológico (306821/2015-0 and 307443/2015-9), and Coordenaçaõ de Aperfeiçoamento de ́ Superior. The authors are also thankful to the Pessoal de Nivel Grupo de Materiais Inorgânicos do Triângulo, a research group supported by FAPEMIG (APQ-00330-14).
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