G. Estiu‡
a,
G. Ferraudi*b,
A. G. Lappin*a,
G. T. Ruiz*c,
C. Vericatc,
J. Costamagnad and
M. Villagrand
aDepartment of Chemistry, Univ. of Notre Dame, Notre Dame, IN 46556, USA
bDepartment of Chemistry, Radiation Research Building, Univ. of Notre Dame, Notre Dame, IN 46556, USA. E-mail: ferraudi.1@nd.edu
cInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA, UNLP, CCT La Plata-CONICET), Diag. 113 y 64, Sucursal 4, C.C. 16, B1906ZAA La Plata, Argentina
dFaculty of Chemistry and Biology, Universidad de Santiago de Chile, Santiago, Chile
First published on 8th October 2014
The photochemical reactions of the Ni(II) annulene complex, [NiII([5,7,12,14]-tetra methyl dibenzo[2,3-b:2,3-b,i][1,4,8,11]tetraaza[14]annulenate)], grafted into a poly(isobutylene-alt-maleate) backbone were investigated in aqueous media. The grafted Ni(II) complex becomes soluble in aqueous and organic solvents where the strands form aggregates with medium-dependent shapes. Irradiation of the polymer at 532 or 351 nm produce charge-separated macrocyclic pendants, CS, with a lifetime τ ∼ 30 ns. CS reacts with electron donors and acceptors before it decays with a lifetime τ ∼ 1 μs. In parallel to the decay of CS, an excited state-excited state annihilation process gives rise to luminescence whose spectrum spans wavelengths shorter than the wavelength of the irradiation, λex > 500 nm. Theoretical calculations were carried out with the aim of understanding the morphology and structures of strand aggregates, to confirm the nature of reaction products and to account for the spectroscopic and photochemical properties of the Ni(II) pendants. The endothermic reduction of CO2 to CO by S(IV) species was used as a test of the Ni(II) complex's ability to photocatalyze the reaction. In the photoprocess, the Ni(II) complex fulfills the double role of antenna and catalyst.
SO2(g) + CO2(g) → SO3(g) + CO(g) | (1) |
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Scheme 1 Structure of the free complexes (top) and average structures of the poly[NiII(tmdbzTAA)]n polymers with C, N, O, Ni; H atoms are not shown. |
This energetic deficit corresponds to a wavelength for photo-activation of 642 nm. In aqueous solution at pH = 8, the appropriate reaction, is also disfavoured but with a smaller energy deficit, ≥70 kJ mol−1, eqn (2).
SO2(aq.) + CO2(aq.) + H2O → SO42−(aq.) + CO(g) + 2H+(aq.) | (2) |
Consequently, this reaction can be photo-activated with visible light with a catalytic reagent that incorporates an absorption antenna. Under these conditions, poly(NiII[tmdbzTAA])n, becomes a catalyst of practical importance, useful for storage of energy.
Different procedures for the grafting of transition metal complexes into preformed polymers have been described in the literature.24 One, leading to the formation of covalent bonds between the polymer and the ligand, has been applied to the syntheses of poly(NiII[tmdbzTAA]).
The relationships of [NiII(tmdbzTAA-CO⋯)] pendants to isobutylenemaleate groups were confirmed on the basis of the C, N, H, elemental analysis. Based on the weight-average molecular weight, Mw = 104, of the poly(isobutylene-alt-maleic anydride) and the Ni contents in poly(NiII[tmdbzTAA]), n = 1, 2, it was estimated that respectively have 5 and 2.5 isobutylenemaleate units per [NiII(tmdbzTAA-CO⋯)] pendant, Scheme 1. The structure and morphology of poly(NiII[tmdbzTAA])n were established on the basis of 1H-NMR, FTIR, UV-Vis spectroscopic and AFM, TEM experimental observations. The TEM and AFM experimental observations are communicated together in the ESI.† For the sake of simplicity, only conclusions about the structure and morphology of poly(NiII[tmdbzTAA])n are briefly described at the beginning of the Results section. The 1H-NMR spectrum of poly(Ni[tmdbzTAA])n in dimethylsulfoxide-d6 shows resonances at 6.76–6.88, 4.90 and 2.00 ppm (labeled a in Fig. S1a†). They are assigned to the C6H4, CH3 and CH groups in the macrocyclic ligand by comparison to the literature H-NMR spectrum of NiII[tmdbzTAA]. Moreover, resonances observed in a 1.0 to 2.8 ppm region, i.e., those labeled b in Fig. S1a,† have been assigned to the polymer backbone. In the FTIR spectrum of the solid poly(NiII[tmdbzTAA])n, n = 1, 2, various frequencies were assigned to characteristic vibrations of the [NiII(tmdbzTAA-CO⋯)]pendants, Fig. S1b and c,† by comparison to the FTIR spectrum of the NiII[tmdbzTAA]. In addition to the vibrations of the pendent macrocycle, the spectrum also displays ν(CH), ν(C
O) and carboxylate vibrations, typical of the polymer backbone. These experimental observations can be interpreted in terms of average structures of poly(NiII[tmdbzTAA])n, n = 1, 2, shown in the Scheme 1.
In other experiments, a solution containing 20 to 30 mg of poly(Ni[tmdbzTAA])2 in 20 cm3 of a 20% or 30% (v/v) CH3CN in H2O mixed solvent was deaerated with the 1:
1 molar of CO2 and SO2 and irradiated with 520 nm light for a period of 24 h. A product yield, ϕ = 0.3, was calculated from the mols of CO produced and the number of 520 nm photons absorbed by the solution.
The CO product was also detected in the white light photolysis of a solution where 20 to 30 mg of poly(Ni[tmdbzTAA])2 were added to 20 cm3 of a solution containing 0.1 M in NaHSO3, 0.1 M in Na2CO3 and equilibrated under 1 atm of CO2. In this condition, the CO was produced in a smaller yield than in the condition of the white-light photolysis described above.
When the reactions are carried out in the absence of CH3CN at pH = 8, CO is also generated but the reaction extent is limited by precipitation of the catalyst when the pH decreases below pH = 6. The precipitated catalyst can be redissolved and reactivated in aqueous media at pH = 8.
The pH-induced morphological changes are also reflected in the cyclic voltammetry of [NiII(tmdbzTAA-CO⋯)] pendants. A saturated solution of poly(NiII[tmdbzTAA])2 in deaerated DMF, where the deprotonated globular aggregate is the most abundant, was used in a study of the nickel(II) pendants redox behavior. The voltammetric response consisted of two irreversible peaks at +0.75 V and +1.20 V vs. Ag/AgCl in the anodic region and an essentially irreversible peak at ca. −1.7 V in the cathodic region, Fig. S5.† The peaks at +0.75 V and +1.20 V are very close to those reported for the cyclic voltammogram of [NiII(tmdbzTAA)],25–28 and have been assigned to the oxidation of the macrocyclic ligand by the successive removal of 1 and 2 electrons. A peak at −1.7 V vs. Ag/AgCl compares very well with that assigned to the nickel(II)/(i) couple in the [NiII(tmdbzTAA)] complex.26
The [NiII(tmdbzTAA-CO⋯)] protonation was followed by recording voltammograms for Ar-deaerated solutions of the polymer containing 4.9 × 10−4 M [NiII(tmdbzTAA-CO⋯)] pendants in a 20% (v/v) H2O in CH3CN mixed solvent with a pH = 4. The first voltammogram was recorded 2 min after the aqueous and CH3CN solutions were mixed in the manner described in the spectroscopic study of the [NiII(tmdbzTAA-CO⋯)] protonation. Additional one-sweep voltammograms were successively recorded at various intervals over a period of 24 h. The irreversible waves at +0.6 and +1.1 V vs. SCE assigned to the successive 1 and 2 electron oxidations of the macrocyclic ligand, shifted to more positive potentials with time. The wave at +0.6 V becomes vanishingly small in those voltammograms recorded with solutions aged more than 1400 min. The displacement of the +1.1 V wave is less pronounced. The changes in the position and the intensity of the wave occur on the same timescale as changes in the optical spectrum of [NiII(tmdbzTAA-CO⋯)] pendants due to the ligand protonation.
Broad irreversible waves were observed in the 0 to −0.75 V cathodic sweep. They move within this range of potential and become also more defined with time. Some of the waves can be associated with covalently linked intra- and inter-strand pendant dimers formed by a one electron oxidation of the annulene ligand. A related electrochemical formation of dimers has been observed by several groups investigating the electrochemistry of [M(tmdbzTAA)], M = Cu and Ni, complexes.26–28 The dependence of the cyclic voltammograms on time and pH suggests that the most important processes responsible of the observed changes in the voltammograms are the formation of dimers (or oligomers) of the Ni(II) pendants in conjunction with morphological transformations caused by the protonation of the polymer.
The absorption bands were assigned on the basis of the results of TDDFT calculations in acetonitrile, using the structure in Fig. 2, which includes the annulene pendant and a 6-acetyl group that models the effect of the polymer backbone on the electronic structure of the complex, Scheme 1. Using this model, absorption bands are calculated at 573, 373 and 315 nm (with relative intensities 0.04:
0.8
:
0.04), together with shoulders at 462 and 411 nm. A feature that immediately emerges from the electronic structure calculations is that the molecular orbitals have significant contributions from both the metal and ligand moieties. Labels for the excited states are based here on the two orbitals involved in the electronic transition, Scheme 2. LMCT, ππ′ and dd′ subscripts have been used to indicate the respective sense of the repolarizations rather than the more effective displacements of charge occurring in complexes with a higher ionic character.
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Fig. 2 Molecular orbitals involved in the UV-vis electronic transitions. The optimized structure of the model compound used in the TDDFT calculations is shown at the top of the figure. |
In addition to the spectroscopic assignments, these calculations were also used to help interpret photochemical processes. Scheme 2 shows the excited states populated by various light excitations. The strong band at 394 nm (calculated 373 nm) is associated with a transition from the HOMO to the LUMO + 1. It brings about a partial transfer from d-electron density to the delocalized π*′ annulene system. Both orbitals involved in the transfer have π electron density, justifying the observed intensity. The excited state populated by this electronic transition is labeled (HOMO)1(LUMO)1-MLCT to indicate some degree of charge transfer character in the electronic transition, Fig. 2. Superscripts indicate electrons in the orbital. Electronic transitions from the HOMO − 1 to the LUMO and from the HOMO − 6 to the LUMO (Fig. 2) give rise respectively to the band at 585 and the shoulder at 411 nm. The electronic transitions involve a displacement of the charge that mainly affects the Ni center. They can be regarded as d → d′ transitions with the d orbitals strongly mixed with the ligand π system. The exited states are named as (HOMO − 1)1(LUMO)1-dd′ and (HOMO − 6)1(LUMO)1-dd′ for future reference. The peak at 300 nm (calculated at 315 nm) has origin in an electronic transition from the HOMO − 1 to the LUMO + 3. While the electronic density in the HOMO − 1 is principally centered on the phenylene groups, it is shifted in the LUMO + 3 over the 5,7-dimethyl-6-carbonyl-4,8-diazapenta-4,6-dienyl group of the annulene ligand. The electronic transition at 300 nm causes a major change in the π density of the ligand and the excited state populated by the transition is then labeled (HOMO − 1)1(LUMO + 3)1-ππ*. Another transition to the LUMO + 3 but this time from the HOMO − 6 orbital gives rise to the shoulder at 462 nm. The excited state has a MLCT character and is labeled (HOMO − 6)1(LUMO + 3)1-MLCT.
In the 351 nm irradiation of deaerated solutions of poly(NiII[tmdbzTAA])2 at either pH = 8 or 10, the spectrum, recorded immediately after the laser flash, t ≤ 15 ns, is shown in the bottom frame of Fig. 3 along with subsequent transient spectra recorded over a millisecond time domain using different delays from the laser flash. In the solution buffered at pH = 10, the changing spectroscopic features reveal that an intermediate, referred to as CS (charge separated), is formed in a short period of time after the laser irradiation, i.e., t ∼ (30 ± 5) ns, and decays on a longer time scale. Moreover, a bleach of the solution in the region between 500 and 650 nm remains at the end of the decay process. This bleach is consistent with a long term photoinduced transformation of the polymer, so that the [NiII(tmdbzTAA-CO⋯)] pendants are not returned to the condition they had prior to flash irradiation. Oscillographic traces following the decay of the photogenerated absorbance at various wavelengths exhibited two well-defined kinetic steps. Each step was fitted with χ2 ≥ 0.978 to exponentials with rate constants: kf = 3.6 × 107 s−1 for the formation of CS and ks = 9.5 × 105 s−1 for the decay.
The photochemical behavior of poly(NiII[tmdbzTAA])2 was also investigated in a 20% (v/v) H2O in CH3CN mixed solvent acidified topH = 4 with HClO4. Solutions were aged for 48 h to insure that the slow protonation of the pendants was complete. A transient spectrum with λmax = 438 nm was promptly generated by the flash irradiation at 351 nm, Fig. 3 top frame. This initially generated spectrum experiences a small change that shifts λmax from 438 nm to 427 nm in a 102 ns period after the flash. This spectroscopic transformation, investigated between 380 nm and 450 nm over a 1 μs period, must be assigned to the formation of a transient species from the one promptly formed during the laser flash irradiation. Following formation of this transient, the associated spectrum decays in a time domain of microseconds. Logarithmic plots, lnξ vs. time, are linear and a rate constant, k = 6.6 × 105 s−1, is calculated from the slope of the line.
Spectroscopic transformations during photolysis at 351 nm of poly(NiII[tmdbzTAA])2 solutions buffered at pH = 4, 8 or 10 are attributed to the formation of CS. This charge-separated species is formed from the excited state that has weak charge transfer character, Scheme 2. Assignment as a charge-separated species is based on the redox properties of the pendants revealed in cyclic voltammetry and the observation of related long-lived species in the photolysis of MII[tmdnTAA], M = Ni and Cu.25 Furthermore, the character of the charge-separated species can be tested by making evident their electron donating and electron accepting reactivity under appropriate thermochemical and kinetic conditions. To confirm the formation of such charge-separated pendants in the 351 nm irradiation of poly(NiII[tmdbzTAA])2, the polymer was flash irradiated in solutions that also contained electron and hydrogen atom donors, i.e., 0.1 to 1.0 M 2-propanol, 0.1 M NaI (E° = 1.42 V vs. NHE for the I˙/I− couple), 0.1 M tEA and 0.1 M tEOA (tEOA = triethanolamine, E° = 0.82 V vs. NHE for the tEOA˙+/tEOA couple). The electron-donating reactivity of the charge-separated pendants was investigated using O2 (E° = −0.33 V vs. NHE for the O2/O2˙− couple) as the electron acceptor.
A common transient spectrum with λmax = 390 nm is generated in the presence (∼0.1 M) of tEA, tEOA or 2-propanol, Fig. 4, that has a close resemblance to the spectrum of [NiI(tmdbzTAA-CO⋯)]− pendants generated with the eaq.− in pulse radiolysis (vide infra). Accordingly, the spectrum of the products photogenerated in the presence of tEA, tEOA or 2-propanol is attributed to the formation of reduced [NiI(tmdbzTAA-CO⋯)]− pendants when CS reacts with electron donors. In the reaction with tEA, the formation of [NiI(tmdbzTAA-CO⋯)]− pendants is seen as a process with a pseudo-first order kinetics, kpf = 1.3 × 107 s−1. Because 0.1 M tEA was used in the experiment, the rate constant for the second order rate law would be k = kpf/0.1 = 1.3 × 108 M−1 s−1. The decay of the [NiI(tmdbzTAA-CO⋯)]− pendants takes place with a lifetime τ = 2.3 μs. No reaction was observed with NaI.
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Fig. 4 Transient spectra recorded when a deaerated solution of poly(NiII[tmdbzTAA])2 buffered at pH = 8 and containing 0.1 M tEA was irradiated at 351 nm. |
The electron-donating reactivity of CS was investigated in aqueous solutions containing 2.2 × 10−4 M [NiII(tmdbzTAA-CO⋯)] pendants, saturated under 1 atm of O2 ([O2] ∼5.2 × 10−4 M), buffered at pH = 8, and flash irradiated at 351 nm. Transient spectra recorded in a 30 ns to 1 μs time domain were nearly identical to those observed with deaerated solutions. However, different kinetics of the slow step in the decay of the transient spectrum were observed in aerated and deaerated solutions. Instead of the linear plots, lnξ vs. time, calculated for the decay of the transient absorbance in deaerated solutions, the plots ξ−1 vs. time are linear in the oxygenated solutions. Assuming a second order kinetics on the basis of these results, a ratio of the rate constant to the extinction coefficient, k/ε = 6.3 × 105 cm−1 s−1, was calculated from an oscillographic trace recorded at λob = 410 nm where the initial change in the absorbance is ΔA0 = 0.094.
The transient generated in a time scale t < 1 μs when poly(NiII[tmdbzTAA])2 is flash irradiated at 532 nm is the same CS intermediate generated in the 351 nm photolysis of the polymer. This assignment is based on the similarity of the transient spectrum recorded when the polymer is respectively irradiated at 532 and 351 nm and on the observation that transients generated in either case exhibit the same chemical reactivity toward electron donors and acceptors. It is possible to examine the reactions of the CS photogenerated with 532 nm light using a more extensive range of electron donors and acceptors than those used in the 351 nm irradiations. These redox active species include the electron donors phenol, tEOA, and ferrocene and the electron acceptors CoIII(acac)3, CuII(acac)2 and CuII(β-alanine)2. Flash irradiation of poly(NiII[tmdbzTAA])2 deaerated solutions containing 2.2 × 10−4 M [NiII(tmdbzTAA-CO⋯)] pendants and an electron donor, i.e., 1.0 × 10−2 tEOA, 1.0 × 10−2 M or 1.0 × 10−1 M phenol, produces spectra where two new absorption bands, λmax = 375 and 580 nm, and a bleach of the solution at ∼500 nm are observed 100 ns after the flash. A third absorption band with λmax = 430 nm observed when the photolyzed solution contained phenol is characteristic of the phenoxide radical reported in the literature,29 and is attributed to the product of the CS reduction by phenol, Fig. 6.
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Fig. 6 The 532 nm flash irradiation of a poly[NiII(tmdbzTAA)]2 solution containing 0.1 M phenol produced the spectrum shown with an absorption band at λmax = 430 nm assigned to the phenoxide radical. |
In the presence of oxidants 1.0 × 10−2 M CoIII(acac)3, 1.0 × 10−2 M CuII(acac)2 or 1.0 × 10−2 M CuII(β-alanine)2, flash irradiation of poly(NiII[tmdbzTAA])2 in deaerated solutions containing 2.2 × 10−4 M [NiII(tmdbzTAA-CO⋯)]pendants produce on a 12 μs time scale, a transient that is stable on the 6 ms time scale. The same spectrum with maxima at 400 and 600 nm was recorded with any of the electron acceptors using delays from the flash equal to or longer than 30 μs, Fig. 7. It is tentatively assigned to the ligand oxidized species NiII[(tmdbzTAA˙+)-CO⋯].
When N2O-saturated solutions of poly(Ni[tmdbzTAA])2 containing 1.0 × 10−2 M NaHCO2 or 2-propanol and 2 × 10−4 M [NiII(tmdbzTAA-CO⋯)] pendants are pulse radiolyzed, Fig. 8a, CO2˙− and (CH3)2C˙OH radicals are respectively produced. Both of the radiolytically prepared radicals react with poly(Ni[tmdbzTAA])2 generating spectra with absorption bands having λmax ≤ 475 nm Fig. 8a. Oscillographic traces showing the growth of the spectra at λob ∼ 400 and 475 nm and under the regime of a first-order or pseudo-first-order kinetics were fitted with χ2 > 0.9900 to single exponentials. Different rate constants, k = 1.7 × 104 s−1 and k = 8.6 × 103 s−1, were calculated for the respective reactions of the CO2˙− and (CH3)2C˙OH radicals. Minimal differences were observed between the spectra of the products respectively generated by the CO2˙− and (CH3)2C˙OH reactions with the pendants. These transient spectra cannot be assigned to [NiI(tmdbzTAA-CO⋯)]− products because reduction potentials of the CO2/CO2˙− and (CH3)2CO/(CH3)2C˙OH couples are insufficiently negative to drive the reductions of the metal center or other groups in the backbone. Therefore, the spectra shown in Fig. 8a must be associated with species produced either by the coordination of the radicals to the metal center or to the addition of the radicals to the annulene ligand. Calculations of the absorption spectrum of these potential products support the assignments of the transient spectra to chromophores that are adducts of the radicals to the annulene macrocycle, eqn (3) and (4).
![]() | (3) |
![]() | (4) |
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Fig. 8 Spectra recorded in pulse radiolysis when poly(NiII[tmdbzTAA])2 was reduced: (a), by CO2˙− (○) and (CH3)2C˙OH (□) and, (b), by the combined actions of eaq.−, and (CH3)2C˙OH. An Inset to (a) and (b) are respectively the optimized structures of the eqn (2) and (3) products. Delays from the radiolytic pulse used recording the spectra are: (○) 500 μs, (□) 300 μs in (a) and (◊) 15 μs, (□) 2.6 ms in (b). Other experimental details are given in the text. |
Optimized structures obtained from the calculations are shown in the insets to Fig. 8a and b respectively, and show that the adducts of the CO2˙− and (CH3)2C˙OH radicals to the annulene ligand can be regarded as Ni(II)–ligand radical moieties. The spectra calculated for these Ni(II)–ligand radical moieties exhibit the following bands: 770 (736) nm, 650 (630) nm and 573 (563) nm (intraligand charge transfer) and 448 (449) nm, 386 (396) nm (π → π* transitions) for the respective adducts of CO2˙− and (CH3)2C˙OH (numbers in parenthesis).
In the radiolysis of N2-deaerated aqueous solutions of poly(Ni[tmdbzTAA])2, the eaq.− and (CH3)2C˙OH reactants are generated when (CH3)2CHOH is the scavenger of the H˙ and OH˙ radicals produced by the solvent irradiation. The (CH3)2C˙OH and eaq.− are formed in a time scale shorter than 1 μs in the radiolysis of a solution containing 1.0 × 10−4 M [NiII(tmdbzTAA-CO⋯)] pendants and 1.0 × 10−2 M (CH3)2CHOH. Spectra recorded with delays as short as 15 μs, Fig. 8b (◊), exhibited bands with λmax = 390 nm and 460 nm. The intensity and positions of these peaks are considerable different of those (λmax = 405 and 472 nm) observed in the spectrum of the species generated by the (CH3)2C˙OH radicals alone, Fig. 8a (○). They were attributed to a product of the fast reaction of the pendants with eaq.−. The growth of this product spectrum, nearly ∼40 times faster than the observed reaction between the pendants and (CH3)2C˙OH, sets apart the respective reactions of the (CH3)2C˙OH radical and eaq.− with [NiII(tmdbzTAA-CO⋯)] pendants. A pseudo-first order rate constant, k = 3.7 × 105 s−1 was calculated from the absorbance growth at λob = 390 nm for the eaq.− reaction. Consistent with the reduction of the [NiII(tmdbzTAA-CO⋯)] pendant, a good agreement was found between the spectrum in Fig. 8b (◊) and one calculated from theory for the optimized geometry of [NiI(tmdbzTAA-CO⋯)]− in water solution. The latter shows the most intense peak at 386 nm generated in a d → π* transition, with the d orbital heavily mixed with ligand orbitals of π symmetry. Transitions of lower intensities, at 397 nm, 433 nm, 461 nm and 503 nm shared similar assignments. A peak of lower intensity at 675 nm is associated with d → d′ transitions that gain intensity through mixing with ligand π orbitals.
On the basis of the previous observations, the formation of [NiI(tmdbzTAA-CO⋯)]− in the reaction of the pendants with eaq.− can be represented by the eqn (5).
![]() | (5) |
The Ni(I) product of eqn (5) is unstable and decays in a longer time scale. A spectrum recorded after the decay of this product, Fig. 8b (□), was the same as the spectrum of the adduct of the (CH3)2C˙OH radical to the annulene ligand, Fig. 8a (□). Oscillographic traces recorded at 380 nm showed that the rate of decay of the [NiI(tmdbzTAA-CO⋯)]− pendants is kinetically of a first order in the transient concentration. A rate constant, kd = 7.7 × 103 s−1, was calculated from the fitting of the oscillographic traces to an exponential ξ = exp(−kdt).
A particular feature of the luminescence spectrum is that the structured luminescence spans a broad range of wavelengths which encompass wavelengths shorter than those used for the irradiation of the polymer, λexc = 532 (flash irradiations) and 580 nm (steady state irradiations), Fig. 9a. Therefore, emitted photons have a higher energy than those used for the excitation of the chromophores. The dependence of the luminescence on the exciting wavelength is reflected in the excitation spectrum, Fig. 9b. It shows that luminescence in the spectroscopic region of the short wavelengths is also induced when the polymer is irradiated at ∼330 nm. The upper excited states populated by the 330 nm light must decay to the emissive excited state reached also from the lower lying excited state produced by the 530 or 580 nm light. To explain the formation of the upper emissive excited state at expenses of the lower energy excited states, the latter must acquire additional energy. Because the phenomenon is observed in the irradiations with low and high intensity light, the acquisition of additional energy is likely to occur through an excited state-excited state annihilation process.
The reduction of CO2 in eqn (1), as written is thermodynamically disfavoured by 186.36 kJ mol−1. In aqueous solution at pH = 8, the appropriate reaction, eqn (2), is similarly disfavoured by 70 kJ mol−1. This energetic deficit corresponds to a wavelength for photo-activation of 642 nm.
Studies of photocatalytic behavior carried out with poly(Ni[tmdbzTAA])2 at pH = 8, show high turnover numbers with visible light. The product yield measured at 365 nm is 0.3.
One consequence of the reaction that takes place, eqn (2), is that the pH of the solution gradually decreases and photocatalysis stops when the pH drops below 5, the result of precipitation of the polymer. Photocatalytic activity can be restored by re-dissolution of the polymer at pH 8.
The focus of this study is an examination of the basic morphology of the polymer to understand the effects of pH and the nature of the photocatalytic species.
Aggregation of multiple strands with the conformations described above explain the shapes seen in the micrographs displayed in Fig. S3 and S4.† We also managed to generate a initial 3-strand geometry able to keep the strands together during the MD, Fig. 10c and see a movie showing the oscillatory motions of three strand fragments. When more than one strand is present, the main interactions that glue them together are the interstrand H-bonds between the carboxylate groups, as shown in the figure, although some inter strand H-bonds between protons of carboxylic groups and the annulene N atoms are also observed. Although these results do not provide a quantitative assessment, they allow the potential interactions that can contribute to the observed morphology to be visualized.
Among the factors influencing the change of shape from elongated to globular, a different balance of inter- to intra-strand interactions as shown in Fig. 10a–c has to be considered. Carboxylic acid and carboxylate groups are prone to form H-bond interactions, potentially leading to linear multistrand assemblies when formed between different strands, as shown in Fig. 10c for the case of 3 chains fully protonated (partial deprotonation will reinforce the H-bond interactions). Inter- and intra-strand interactions between protonated carboxylates and deprotonated annulene pendants are more prone to stabilize non-linear structures. As the number of inter-strand interactions increases with concentration, a larger percentage of the carboxylate-annulene pendant interactions favours the formation of non-linear shapes. While the basic medium produces a highly charged backbone, ligand-deprotonation of the [NiII(tmdbzTAA-CO⋯)] neutralizes the ionic charge (gradually from +2 to +1 and to 0) making the pendants highly hydrophobic. In such conditions, a globular aggregation of strands, as shown in Fig. S4a,† where much ionic charge is directed towards the solvent and the pendants are forced into the interior of the aggregate is favoured over more linear shapes, as shown in Fig. S3.†
![]() | (6) |
2[Niii(tmdbzTAA-CO˙⋯)]+ → [NiII(tmdbzTAA-CO˙⋯)]22+ | (7) |
Oxidation of the complex at 1.2 V must be related to the removal of the second electron, eqn (8).
![]() | (8) |
It is possible to attribute the irreversibility in the removal of this second electron to the coupling of the reaction in eqn (8) with the redox process,31 although it is also possible that the removal of the second electron is coupled to the introduction of a keto group into the macrocyclic ligand, as it is illustrated in eqn (9).
![]() | (9) |
A similar product with a keto group is formed in the air oxidation of the [CoII(tmdbzTAA)] complex.32
The one-electron electrochemical reduction of [NiII(tmdbzTAA)] to [NiI(tmdbzTAA)]−, like the reduction of [NiII(tmdbzTAA-CO⋯)] pendants is an irreversible processes. The [NiI(tmdbzTAA-CO⋯)]− pendants that are produced by the eaq.− in pulse radiolysis, were examined to compare with the products of the electrochemical reduction of the pendants. Calculations show that the pulse radiolysis product has a large fraction of the charge localized on the metal center and to some extent, the product can be regarded as a nickel(I) complex. The instability of [NiI(tmdbzTAA-CO⋯)]− shown in the pulse radiolysis experiments where it is seen to decay with a lifetime τ = 1/kd ∼ 100 μs is consistent with the irreversibility of the reduction process in cyclic voltammetry.
Further consideration must be given to the instability of the [NiI(tmdbzTAA-CO⋯)]− and [NiII(tmdbzTAA-CO⋯˙)]+ pendants if they are viable intermediates in catalyzed processes, such as the photocatalyzed reduction of CO2 where S(IV) species are used as sacrificial reactants. Such reaction intermediates must be scavenged, thereby regenerating the [NiII(tmdbzTAA-CO⋯)] pendants to allow the catalysis to continue to high turnover numbers. Moreover, the scavenging rate must be fast enough to prevent the [NiI(tmdbzTAA-CO⋯)]− and [NiII(tmdbzTAA-CO⋯˙)]+ from entering the reactions that irreversibly degrade the poly(Ni[tmdbzTAA])2. One possible mechanism has the CO product stabilizing the Ni(I) species.
While the (HOMO − 1)1(LUMO)1-dd′ and (HOMO)1(LUMO)1-MLCT excited states both result in the formation of the CS, the luminescence of poly(NiII[tmdbzTAA])2 is more efficient when it is irradiated at 532 or 580 nm. The spectroscopic features of the luminescence are a broad and structured spectrum that spans significantly over wavelengths shorter than the exciting wavelengths, λex = 532 and 580 nm. The structured emission band is characteristic of an emitting excited state with considerable annulene ligand-centered character, and in order to explain the emission of light at wavelengths much shorter than the wavelengths of the absorbed light, an energy accretion mechanism must be invoked. Because the luminescence is seen both in the 532 nm flash irradiations with a high light intensity and 580 nm steady state irradiations with a low light intensity, it cannot be associated with the sequential absorption of two photons. The experimental observation can be rationalized by a mechanism that involves the excited state-excited state annihilation to produce an emissive (HOMO − 6)1(LUMO)1-dd′ excited state, eqn (10) and (11).
![]() | (10) |
![]() | (11) |
Similar excited state-excited state annihilation processes have been observed in the photolysis of Re(CO)3(1,10-phenanthroline)+ bonded to poly-(vinylpyridine)600 and a polyimide containing pendant AlIIIphthalocyaninetetrasulfonate groups, [HOAlIII(pc)(SO3−)3(SO2N<)]3−.2,33 If energy migration through the bundle brings two electronically excited pendants to the contact distance, an exciplex could be formed before the products of eqn (10). An alternative mechanism for the excited state – excited state annihilation involves Förster–Dexter34 energy transfers. Energy migration through the bundle needs only to bring the excited pendants to a distance where the rate of energy transfer equals the rate of spontaneous decay of the excited state.
Inspection of the excitation spectrum in Fig. 8 reveals that the excited state annihilation process, eqn (9) and (10), justifies the induced emission when λex = 532 nm but does not account for the intense luminescence observed when poly(Ni[tmdbzTAA])2 solutions are irradiated at wavelengths λex < 400 nm. Given the ∼330 nm position of the maximum in the excitation spectrum, there must be an upper excited state that converts with a significant yield to the emissive (HOMO − 6)1(LUMO)1-dd′ excited state populated by the excited state annihilation process, eqn (9) and (10) and Scheme 2. On the basis of the calculated energies of the excited states, it is likely that the upper excited state is the (HOMO − 1)1(LUMO + 3)1-dd′. This assignment is consistent with the assignment of the ∼330 nm shoulder in the absorption spectrum of the pendant to the electronic transition from the ground state to the (HOMO − 1)1(LUMO + 3)1-dd′, Fig. 2. No evidence was found that the CS intermediate is formed in the decay of the (HOMO − 1)1(LUMO + 3)1-dd′.
Intermediates in the photolysis of M(tmdnTAA), M = Cu(II) and nickel(II), were tentatively assigned as charge separated species in a previous study,25 and based on the experimental observations presented in this work, a similar assignment is made for CS. Indeed, the dual reactivity of the intermediate with electron donors and acceptors can only be rationalized if CS has a structure possessing some kind of charge separation. Such charge separation can be that of a biradical or a nickel(I)–ligand radical intermediate, [NiI[(tmdbzTAA˙+)-CO⋯]. A biradical structure requires that different charge be localized in different groups of the annulene ligand with little if any change in the electronic density of the metal center. In contrast, the metal center in [NiI[(tmdbzTAA˙+)-CO⋯] has increased its charge at expenses of the ligand. The transient spectra recorded when CS reacts with electron donors are identical to the spectrum of the [NiI[(tmdbzTAA)-CO⋯]− produced when [NiII(tmdbzTAA-CO⋯)] pendants are reduced by the eaq.− in pulse radiolysis, and so [NiI(tmdbzTAA-CO⋯)]−, is formed in the reduction of CS. In contrast, reduction of a biradical structure will form a ligand–radical instead than the [NiI[(tmdbzTAA)-CO⋯]− complex, justifying the assignment of a nickel(I)–ligand radical structure to CS.
Footnotes |
† Electronic supplementary information (ESI) available: Fig. S1 with spectroscopic properties of poly(Ni[tmdbzTAA])2. (a): NMR spectrum in dimethylsulfoxide-d6 and (b and c): FTIR in solid phase. Fig. S2 with GC chromatogram of photolysis products of the CO2 reduction. Fig. S3 with TEM and AFM observations of poly(Ni[TMBzTAA])2 in aqueous CH3CN. Fig. S4 with TEM and AFM observations of poly(Ni[TMBzTAA])2 in basic solutions. Fig. S5 with voltammetric responses of poly(Ni[TMBzTAA])2 in N,N′-dimethylformamide. A movie (3chains.wmv) with the molecular mechanics calculation of the motions of three strands fragments. See DOI: 10.1039/c4ra08177c |
‡ This work is dedicated to G. E who passed away on 5/9/2014. |
This journal is © The Royal Society of Chemistry 2014 |