Article pubs.acs.org/IC
Excited States of Triphenylamine-Substituted 2‑Pyridyl-1,2,3-triazole Complexes Gregory S. Huff,† Warrick K. C. Lo,† Raphael Horvath,‡ Jack O. Turner,‡ Xue-Zhong Sun,‡ Geoffrey R. Weal,† Hannah J. Davidson,† Aaron D. W. Kennedy,† C. John McAdam,† James D. Crowley,*,† Michael W. George,*,‡,§ and Keith C. Gordon*,† †
Department of Chemistry, University of Otago, Dunedin 9054, New Zealand School of Chemistry, University of Nottingham, Nottingham NG7 2RD, U.K. § Department of Chemical and Environmental Engineering, University of Nottingham Ningbo China, 199 Taikang East Road, Ningbo 315100, China ‡
S Supporting Information *
ABSTRACT: A new 2-pyridyl-1,2,3-triazole (pytri) ligand, TPA-pytri, substituted with a triphenylamine (TPA) donor group on the 5 position of the pyridyl unit was synthesized and characterized. Dichloroplatinum(II), bis(phenylacetylide)platinum(II), bromotricarbonylrhenium(I), and bis(bipyridyl)ruthenium(II) complexes of this ligand were synthesized and compared to complexes of pytri ligands without the TPA substituent. The complexes of unsubstituted pytri ligands show metal-to-ligand charge-transfer (MLCT) absorption bands involving the pytri ligand in the near-UV region. These transitions are complemented by intraligand charge-transfer (ILCT) bands in the TPA-pytri complexes, resulting in greatly improved visible absorption (λmax = 421 nm and ϵ = 19800 M−1 cm−1 for [Pt(TPA-pytri)Cl2]). The resonance Raman enhancement patterns allow for assignment of these absorption bands. The [Re(TPA-pytri)(CO)3Br] and [Pt(TPA-pytri)(CCPh)2] complexes were examined with time-resolved infrared spectroscopy. Shifts in the CC and CO stretching bands revealed that the complexes form states with increased electron density about their metal centers. [Pt(TPA-pytri)Cl2] is unusual in that it is emissive despite the presence of deactivating d−d states, which prevents emission from the unsubstituted pytri complex.
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INTRODUCTION The design of molecules for light-harvesting applications requires the balancing of many electronic properties. Platinum(II)1 and ruthenium(II)2 complexes have been studied as dyes for dye-sensitized solar cells and hydrogen production catalysts.3,4 Rhenium(I) complexes have been used in organic light-emitting diodes,5,6 as bioimaging agents7,8 and as CO2 reduction catalysts.9,10 Strong absorption of visible light is often an important goal in designing molecules for these applications, and investigations including steady state and time-resolved vibrational spectroscopy are useful tools in elucidation of the nature and interplay between excited states.11,12 Photoexcitation of these complexes generally involves metalto-ligand charge-transfer (MLCT)-type transitions, which have moderate intensity and may not extend into the visible region.12,13 For instance, [Ru(phen)3]2+ has an MLCT band at 445 nm (ϵ = 17000 M−1 cm−1) in acetonitrile.14 Changing the energies of the metal-based dπ and diimine-based π* orbitals, which act as the electron donor and acceptor in MLCT transitions, is one way of tuning the absorption bands of such complexes.14 It is also possible to append electron-accepting groups such as arylboranes to increase the conjugation length of the molecule and increase the extinction coefficient of the MLCT band.14 © 2016 American Chemical Society
Appending organic chromophores to an inorganic complex results in enhanced absorption and interesting excited-state properties. For example, the covalent linking of coumarin chromophores to [Ru(phen)3]2+ complexes results in increased absorption in the UV region, but emission from the [Ru(phen)3]2+ 3MLCT state is maintained regardless of the excitation wavelength.15 Attachment of other chromophores such as pyrene or 4-piperidinyl-1,8-naphthalimide to ruthenium(II) or rhenium(I) complexes gives enhanced absorption as well as increased excited-state lifetimes due to the triplet reservoir effect.16,17 The absorption spectra of the complexes appear essentially as a combination of the spectra of the parent chromophore and complex. A different approach is to incorporate organic electron-donor units into the ligands of transition-metal complexes, which can bring about additional strong visible absorption bands.18−25 Electron-rich groups, such as triarylamines and thioethers,26,27 give rise to intraligand charge-transfer (ILCT) transitions, which generally have higher extinction coefficients than MLCT transitions.12 Here the donor orbital in an ILCT transition is localized on the organic donor unit, while the acceptor orbital is Received: August 12, 2016 Published: November 14, 2016 12238
DOI: 10.1021/acs.inorgchem.6b01959 Inorg. Chem. 2016, 55, 12238−12253
Article
Inorganic Chemistry a π* orbital as in an MLCT transition. There have been several studies on the effect of substitution of triphenylamine (TPA) as well as the behavior of different conjugated linkers that join the donor to the ligand.23,25,28 In this paper, we present some d6 and d8 transition-metal complexes based on previously studied 2-pyridyl-1,2,3-triazole (pytri) ligands (Figure 1). The pytri ligands are synthesized
The tertiary propargyl alcohol group of 1 was removed with KOH powder in toluene to provide compound 2 in 86% yield. The TPA-pytri ligand was generated from 2, benzyl bromide, and NaN3 using one-pot in situ azide formation-click conditions40,41 at room temperature in 70% yield. Increasing the temperature of the reaction to 65 °C further improved the yield (82%). Compounds 1 and 2 and TPA-pytri were characterized using IR, 1H and 13C NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), and elemental analysis (Supporting Information, Figures S1−S9). In addition, molecular structures of 2 and TPA-pytri (cocrystallized with KOH) were determined by X-ray crystallography (Supporting Information, Figures S10−S13). Metal Complex Synthesis. Ruthenium(II), rhenium(I), and platinum(II) complexes of TPA-pytri ([Ru(TPA-pytri)(bpy)2]2+, [Re(TPA-pytri)(CO)3Br], and [Pt(TPA-pytri)Cl2]) were prepared using the methods previously exploited for generation of the parent pytri complexes (Scheme 2).34 The dichloroplatinum(II) complex [Pt(TPA-pytri)Cl2] was reacted with phenylacetylene, triethylamine, and copper(I) iodide in CH2Cl2. The crude product was purified on a silica gel column and gave the bis(phenylacetylide)platinum(II) complex [Pt(TPA-pytri)(CCPh)2] in moderate yield (59%). [Pt(hexpytri)Cl2] and [Pt(hexpytri)(CCPh)2] were synthesized using analogous methods. All complexes were characterized using IR, 1H and 13C NMR spectroscopies, HRESI-MS, and elemental analysis (Supporting Information, Figures S14− S24). The molecular structures of [Ru(TPA-pytri)(bpy)2]2+, [Re(TPA-pytri)(CO)3Br], and [Pt(TPA-pytri)(CCPh)2] were unambiguously confirmed by X-ray crystallography (Figure 2). The crystal structures of the d6 ruthenium(II) and rhenium(I) complexes ([Ru(TPA-pytri)(bpy)2]2+ and [Re(TPA-pytri)(CO)3Br]) display a distorted octahedral coordination environment about the metal ions. The ruthenium(II) ion of [Ru(TPA-pytri)(bpy)2]2+ is coordinated to two bidentate 2,2′-bipyridine (bpy) ligands and a bidentate TPA-pytri ligand. The divalent charge of the metal ion is balanced by two noncoordinating PF6− anions. The [Ru(TPA-pytri)(bpy)2]2+ cation is chiral. The crystal contains a racemic mixture of Λ and Δ enantiomers, and both enantiomers are present in the asymmetric unit. The Ru−N bond lengths and N−Ru−N angles observed in [Ru(TPA-pytri)(bpy)2]2+ are similar to those reported by us for analogous complexes without the TPA
Figure 1. Structures of the pytri ligands used in the model complexes.
conveniently with “click” chemistry.29−32 A variety of substituents can be incorporated into the complexes through the 1,2,3-triazole ring by using different azide precursors. However, these triazole substituents generally have a very limited effect on the behavior of the complexes formed because the triazole ring acts as an electronic insulator.33−36 The incorporation of electron-donating and -withdrawing groups on the triazole ring has proven to be a poor way of controlling the excited-state behavior of these complexes. Herein we report the synthesis of d6 and d8 metal complexes of a pytri ligand, where the pyridine ring is substituted with a TPA electron-donor unit rather than the triazole ring to give a ligand referred to as TPApytri. TPA substitution of polypyridyl ligands has previously been shown to result in strong ILCT transitions and altered photophysical behavior compared to unsubstituted ligands.12,19,22 A benzyl group is appended to the triazole ring of TPA-pytri, while the pytri complexes without the TPA substituent have either a hexyl group (hexpytri) or a benzyl group (Bnpytri) attached to the triazole ring (Scheme 1 and Figure 1).
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RESULTS AND DISCUSSION Ligand Synthesis. The triphenylamine-substituted 2pyridyl-1,2,3-triazole (TPA-pytri) was synthesized in three steps (Scheme 1). 4-(Diphenylamino)phenylboronic acid and 4-(5-bromopyridin-2-yl)-2-methylbut-3-yn-2-ol37 were reacted using standard palladium(0)-catalyzed Suzuki cross-coupling reaction conditions38,39 and gave compound 1 in 75% yield. Scheme 1. Synthesis of TPA-pytria
Conditions: (i) [Pd(dppf)Cl2], Na2CO3(aq), NaOH(aq), 1,2-dimethoxyethane, 80 °C, 20 h, (ii) KOH powder, toluene, 80 °C, 20 min, (iii) NaN3, benzyl bromide, CuSO4·5H2O, L-ascorbic acid, Na2CO3, 4:1 DMF/H2O, 65 °C, 21 h.
a
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DOI: 10.1021/acs.inorgchem.6b01959 Inorg. Chem. 2016, 55, 12238−12253
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Inorganic Chemistry Scheme 2. Synthesis of TPA-pytri Metal Complexesa
a Conditions: (i) (a) cis-[Ru(bpy)2Cl2], ethanol, microwave (125 °C, 200 W), (b) NH4PF6(aq), (ii) [Re(CO)5Br], ethanol, reflux, (iii) cis[Pt(DMSO)2Cl2], methanol, reflux, and (iv) dry CH2Cl2, copper(I) iodide, phenylacetylene, triethylamine.
substituent.34 While the TPA substituent has little impact on the octahedral coordination environment about the ruthenium(II) ion, it is involved in π-stacking interactions. One of the CH groups of the TPA substituent in the Δ isomer forms a CH···π interaction with the benzyl ring of the Λ isomer (Supporting Information, Figure S25). The rhenium(I) ion of [Re(TPApytri)(CO)3Br] coordinates to a bidentate TPA-pytri ligand, three carbonyl ligands, and a bromide ligand. The carbonyl ligands adopted the expected facial arrangement about the rhenium(I) cation, as was observed for bromidotricarbonylrhenium(I) complexes of pytri, which were characterized by X-ray crystallography.42 The Re−N bond lengths and N−Re−N angles observed in [Re(TPA-pytri)(CO)3Br] are similar to those observed in an analogous complex without the TPA substituent.42 The structure of [Pt(TPA-pytri)(CCPh)2] displayed the expected square-planar geometry about the platinum(II) ion with the metal bound to the bidentate TPA-pytri and two phenylacetylide ligands. Both the benzyl and TPA units of the TPA-pytri ligand are involved in the π−π and CH···π interactions between adjacent molecules (Supporting Information, Figures S26 and S27). Electronic Absorption. The electronic absorption spectra of the molecules studied in this work are presented in Figure 3 and summarized in Table 1. The spectrum of Bnpytri is typical of that for pytri ligands in that there is no visible absorption with the first band appearing at 280 nm (π → π* of the ligand).35 In striking contrast, the TPA-pytri ligand shows a strong absorption band at 350 nm, which is assigned as charge transfer from the TPA π system to the pytri π* system. In the TPA-pytri complexes, this band is red-shifted because of lowering of the pytri π* orbital energies. In the rhenium(I) and platinum(II) complexes, this is clearly seen. In CH2Cl2, the band appears between 398 and 421 nm with a shoulder at higher energy. The pytri complexes lacking the TPA unit (dashed lines, Figure 3) show MLCT bands that match the
wavelength of this shoulder. The same applies to the ruthenium(II) complex, but the bpy-based MLCT band overlaps with the TPA-based ILCT band, giving rise to a single broad band at 412 nm. The largest red shift of the TPApytri ILCT band is observed for [Pt(TPA-pytri)Cl2] (4800 cm−1), which implies that this metal center is the most electron-withdrawing. Resonance Raman spectroscopy and time-dependent density functional theory (TD-DFT) calculations were used to investigate the Franck−Condon states of the molecules. TD-DFT calculations using B3LYP44 are consistent with the absorption band assignments made above. Tables S1−S8 summarize the TD-DFT results and show Mulliken electron density differences for the first 10 electronic transitions. The frontier molecular orbitals for the rhenium(I) complexes are shown in Figure 4 and in the Supporting Information for the other compounds. The MLCT bands of [Re(Bnpytri)(CO)3Br], [Pt(hexpytri)Cl2], and [Pt(hexpytri)(CCPh)2] consist of two separate transitions. The acceptor orbital (lowest unoccupied molecular orbital, LUMO) in each case is a π* orbital spanning the triazole and pyridine rings of the pytri ligand. The highest occupied molecular orbital (HOMO) and HOMO−1 are metal−ancillary ligand dπ orbitals. For [Pt(hexpytri)Cl2], d−d bands are predicted below the MLCT transitions as expected due to the weak-field chloride ligands. For the TPA-pytri complexes, the HOMO is a TPA-based π orbital, which supersedes the dπ orbitals. The LUMO extends somewhat into the phenyl ring pendant to the pyridyl ring, lowering its energy slightly. The ruthenium(II) complexes follow the same pattern, but two bpy MLCT bands are predicted below the pytri MLCT transition. The lowest-energy strong transition for each TPA-pytri complex is predicted to be an ILCT transition from the TPA π orbital to the pytri-based π* orbital. This is what might be expected from the electronic absorption spectra for the rhenium(I) and platinum(II) 12240
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electrolyte-dependent.55−57 The platinum(II) complexes reported here display an irreversible oxidation wave at ca. 1.4 V for those with chloride ligands and at ca. 1.0 V for those with the more electron-donating acetylide ligands. The complexes with the TPA-appended pytri ligands all display an amineassociated oxidation at ca. +1.1 V (a slight anodic shift from the free ligand, consistent with coordination of the pytri component). The reversibility of this process varies with the metal and its ancillary ligands, increases with the scan rate, and shows some solvent dependence. The appearance of the amine oxidation feature before that of the metal (at more cathodic potential) is consistent with the DFT results, which show a TPA-based HOMO. Resonance Raman Spectroscopy. Resonance Raman spectroscopy was used to probe the Franck−Condon states of the TPA-pytri and pytri complexes. Vibrational modes, which mimic changes in the molecular structure that occur during an electronic transition, are enhanced when a laser wavelength coincident with that electronic transition is used to generate Raman scattering. Laser wavelengths between 351 and 458 nm were used to probe the M(L)LCT (MLLCT is metal-andligand-to-ligand charge transfer, see below) and ILCT bands. Nonresonant spectra were recorded at 1064 or 785 nm excitation for comparison. Simulated nonresonant Raman spectra were generated from DFT calculations in order to test the effectiveness of the modeling.26,58 From these calculations, the modes may be assigned, and it is these assignments that are used in the analysis of the resonance Raman spectra. TD-DFT calculations show that for both the M(L)LCT and ILCT bands the same π* acceptor orbital localized on the pyridyl and triazole rings is involved. Therefore, vibrations of these two rings should be enhanced for either type of transition.59 Vibrations of the TPA unit should be enhanced for an ILCT transition, and vibrations of ancillary ligands should be enhanced for M(L)LCT transitions (CC or CO stretches especially). However, the interpretation of the spectra is not always so simple in practice. The resonance Raman spectra of [Re(Bnpytri)(CO)3Br] (Figure S41), which has a pair of MLCT transitions and no ILCT band, show enhancement of several pytri-based modes at 1000, 1026, 1281, 1566, 1585, and 1621 cm−1. All carbonyl stretches are observed in the nonresonant spectrum, but only the symmetric mode at 2030 cm−1 is enhanced when the MLCT band is probed. At wavelengths longer than 375 nm, the resonance Raman spectra are very weak due to loss of resonance and onset of emission. Between 351 and 375 nm, the spectra of [Re(TPApytri)(CO)3Br] (Figure 5) show enhancements of several bands that correspond to bands seen in the [Re(Bnpytri)(CO)3Br] spectra. These are modes associated with the triazole and pyridine rings as well as the pendant phenyl ring (Figure 6a−c) and the symmetric carbonyl stretch. The band at 1618 cm−1 shows significant amplitude on the NPh2 group of the TPA substituent and is yet strongest at shorter wavelengths when the MLCT band is being probed (Figure 6d). However, other modes that are associated with the TPA group are strongest at wavelengths longer than 375 nm, as expected for an ILCT transition. The resonance Raman spectra of [Ru(Bnpytri)(bpy)2]2+ are reported in Figure S42. We showed previously that [Ru(Bnpytri)(bpy)2]2+ has a resonance Raman spectrum nearly identical with that of the prototypical [Ru(bpy)3]2+ with 458 nm excitation.34 Modes associated with bpy (1026, 1039, 1171,
Figure 2. Crystal structures of (a) [Ru(TPA-pytri)(bpy)2]2+(Λ isomer), (b) [Re(TPA-pytri)(CO)3Br], and (c) [Pt(TPA-pytri)(CCPh)2] shown as ORTEP43 diagrams. Only one of the two crystallographically independent molecules of [Ru(TPA-pytri)(bpy)2]2+ and [Pt(TPA-pytri)(CCPh)2] are shown. Counteranions were omitted for clarity. The thermal ellipsoids are displayed at the 50% probability level.
complexes, but for the calculations of the ruthenium(II) complex, the ILCT energy seems to be underestimated compared to the bpy MLCT transition. Electrochemistry. The electrochemistry of the TPA-pytri ligand and its complexes was probed using cyclic and differential-pulse voltammetry in a N,N-dimethylformamide (DMF) solution with Bu4NPF6 as the supporting electrolyte. The results are presented in Table 2 with representative voltammograms in Figures S36−S37. The free TPA-pytri ligand undergoes a quasi-reversible reduction at ca. −2.1 V. The increased reversibility and occurrence at an anodic potential of 300 mV of the pytri34 reduction are consistent with more electron delocalization in the larger triphenylamino system. The appended TPA gives rise to a quasi-reversible oxidation process observed at 1.02 V.45−48 Predictably, the electrochemical behavior of [Re(TPA-pytri)(CO)3Br] closely resembles that of other rhenium tricarbonyl(α-diimine)halide complexes.19,33,47,49−51 Thus, an irreversible one-electron oxidation associated with the ReII/ReI couple is observed at Epa = +1.56 V, and a irreversible, ligand-based reduction wave at Epc = −1.58 V. For the [Ru(TPA-pytri)(bpy)2]2+ complex, three reversible reductions are consecutively assigned to the two bpy and third TPA-pytri ligand by comparison with [Ru(Bnpytri)(bpy)2]2+34 and similar systems.52−54 The chemically reversible RuIII/II oxidation is observed at ca. 1.3 V, consistent with previously reported ruthenium(II) dyes.52−54 Platinum oxidation is typically complex, often irreversible, and solvent- and 12241
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Figure 3. Electronic absorption spectra of the TPA-pytri complexes in CH2Cl2.
This is also predicted by TD-DFT but not easily seen in the UV−vis spectrum (see above). The Raman spectra of [Ru(TPA-pytri)(bpy)2]2+ (Figure 7) are more difficult to interpret because of the presence of several overlapping electronic transitions. The bpy bands are not expected to be perturbed significantly by introduction of the TPA unit. All of the bpy modes from [Ru(Bnpytri)(bpy)2]2+ can been seen in the [Ru(TPA-pytri)(bpy)2]2+ spectra except the 1605 cm−1 mode, which is likely obscured by a very strong TPA-pytri band at 1610 cm−1. As in [Ru(Bnpytri)(bpy)2]2+, these modes are dominant with 448 and 458 nm excitation wavelengths, which shows that the bpy-based MLCT transition is unperturbed by the TPA group. Several TPA-pytri modes (1002, 1143, 1199, 1300, 1393, 1554, 1584, and 1593 cm−1) can, however, be observed at these wavelengths as well. These then become the strongest modes with 407 and 413 nm excitation. Contrary to the TD-DFT calculations for this complex, the resonance Raman enhancement pattern indicates that the ILCT transition is at higher energy than the bpy MLCT transition. The 1476, 1584, and 1615 cm−1 bands are only seen below 407 nm excitation and are associated with the pytri-based MLCT transition around 350 nm as in [Ru(Bnpytri)(bpy)2]2+. The resonance Raman spectra of [Pt(hexpytri)Cl2] (Figure S43) show enhancement of the pyridyl-triazole modes, which are comparable to those of [Re(Bnpytri)(CO)3Br]. Both
Table 1. Photophysical Properties of the pytri Complexes Measured at 298 K in CH2Cl2
TPA-pytri [Re(TPA-pytri) (CO)3Br] [Re(Bnpytri)(CO)3Br] [Pt(TPA-pytri)Cl2] [Pt(hexpytri)Cl2] [Pt(TPA-pytri) (CCPh)2] [Pt(hexpytri)(CCPh)2] [Ru(TPA-pytri) (bpy)2]2+ [Ru(Bnpytri)(bpy)2]2+
λAbs/nm (ϵ/mM−1 cm−1)
λEm/nm
τr/ns
Φr
350 (28.6) 399 (18.3)
453 573
