Electron-Transfer Kinetics within Supramolecular Assemblies of Donor

Article pubs.acs.org/IC

Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium Cluster Peng Luo,† Paul-Ludovic Karsenti,† Gessie Brisard,† Benoit Marsan,*,‡ and Pierre D. Harvey*,† †

Département de chimie, Université de Sherbrooke, Sherbrooke, Quebec J1K 2R1, Canada Département de chimie, Université du Québec à Montréal, Montréal, Quebec H2X 2J6, Canada

‡

S Supporting Information *

ABSTRACT: 9,18,27,36-Tetrakis[meso-(4-carboxyphenyl)]tetrabenzoporphyrinatozinc(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd3(dppm)3(CO)2+ ([Pd32+]; dppm = Ph2PCH2PPh2 as a PF6− salt) with that of 5,10,15,20tetrakis[meso-(4-carboxyphenyl)]porphyrinatozinc(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd32+]x (x = 0−4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV−vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd32+]x assemblies (K14 = 36000 M−1) are slightly more stable than those for TCPP··· [Pd32+]x (K14 = 27000 M−1). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd32+] assembly compared to that for TCPP···[Pd32+]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd32+] → dye•+···[Pd3•+], the time scale for this process is ultrafast in both cases (99% of G

DOI: 10.1021/acs.inorgchem.5b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 6. Computed Positions and Oscillator Strengths (f) of Selected First Electronic Transitions and the Major Contributions for TCPP···[Pd32+] and TCPBP···[Pd32+] by TDDFT (See the SI for the 100st Electronic Transitions) no.

λ, nm

f

739 665 633

0 0.0004 0

HOMO → LUMO (100) HOMO−3 → LUMO (88) HOMO−1 → LUMO (100)

900 798 701 613 613 611 572 540 520 497 466 461 450 449

0 0.0028 0.0003 0.0638 0.1239 0.0471 0.0437 0.2303 0.3575 0.0062 0.0181 0.0029 1.9293 1.2599

HOMO → LUMO (100) HOMO−2 → LUMO (95) HOMO−1 → LUMO (99) HOMO−1 → LUMO+2 (26), HOMO → LUMO+1 (68) HOMO−1 → LUMO+1 (27), HOMO → LUMO+2 (69) HOMO−5 → LUMO (45), HOMO−3 → LUMO (30) HOMO−10 → LUMO (50), HOMO−4 → LUMO (14), HOMO−3 → LUMO (16) HOMO−10 → LUMO (25), HOMO−5 → LUMO (20), HOMO−3 → LUMO (44) HOMO−4 → LUMO (75) HOMO−21 → LUMO (12), HOMO−14 → LUMO (48) HOMO−21 → LUMO (38) HOMO−47 → LUMO (20), HOMO−44 → LUMO (17), HOMO−42 → LUMO (26) HOMO−1 → LUMO+1 (67), HOMO → LUMO+2 (27) HOMO−1 → LUMO+2 (62), HOMO → LUMO+1 (24)

major contributions, %

TCPP···[Pd32+] 1 2 3 TCPBP···[Pd32+] 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Prior to analysis, the unassociated cluster and dyes were examined separately. The transient absorption spectra of [Pd32+] were reported by us previously (see the Supporting Information of ref 8; it is placed again in this SI for convenience). It exhibits positive (bleach) and negative (transient) signals at ∼510 and ∼410 nm, respectively, and the decay is in the short picosecond time scale. The signals are weak compared to that of the porphyrins and does not interfere in any of the spectra. The transient spectra of TCPP (Figure 10) using λexc = 600 nm (i.e., directly in the S1 state) exhibit a species decaying in the nanosecond time scale clearly associated with a triplet state, i.e., T1, of the dye (note that the indicated value is not accurate because of the delay line limited to about 8 ns). This tripletstate species is formed rapidly within the pulse (fwhm =75 fs), so the relative amount of the emissive S1 species (where τF = 2.24 ns) is small despite evidence of a fluorescent species. Upon excitation at λexc = 400 nm (i.e., in the high energy side of the Soret band), three species are depicted. First, the T1 species with nanosecond decay (again its lifetime is inaccurately estimated because of the delay line) is again the dominant species. Second, the species decaying at 1 ps (blue spectrum, middle right; purple decay, bottom right) is the S2 species clearly identified by a comparison of its lifetime with that of 5,10,15,20-tetra-meso-phenylporphyrinatozinc(II) [ZnTPP; τ(S2) ≈ 1.3 ps].39 Finally, the third species decaying at 780 ps (green spectrum, middle right) is very weak, rendering the measurement of its lifetime difficult, and is tentatively assigned to a S1 species (note the that τF is ∼2 ns at 298 K). However, solvent-induced vibrationally relaxed S1 species commonly encountered in ZnTPP-containing chromophores also exist,40 but these nonemissive S1 species decay in the picosecond to tens of picoseconds range. Because of this large difference between the lifetime of such species and the experimental values, this possibility is excluded. It is worth noting that the weakness of the signal makes this species a noninterfering component in this study. Upon generation of the TCPP···[Pd32+]x assemblies using [CO2−]/[Pd32+] stoichiometries of 1:1 and 1:2, time evolution

Figure 9. Experimental UV−vis spectra and oscillator strengths for the 100st electronic transitions for TCPP···[Pd32+] (left) and TCPBP··· [Pd32+] (right) in a MeOH solvent field. The experimental UV−vis spectrum was recorded under 1 equiv of TCPP (or TCPBP) versus 8 equiv of [Pd32+] in MeOH.

Table 7. Relative Percentage of Complexed Dyes versus the [CO2−]/[Pd32+] Ratio [CO2−]/ [Pd32+]

assembly

1:1

TCPP···[Pd32+]4 TCPP···[Pd32+]3 TCPP···[Pd32+]2 TCPP···[Pd32+]

1:2

TCPP···[Pd32+]4 TCPP···[Pd32+]3 TCPP···[Pd32+]2 TCPP···[Pd32+]

% K14, %V 28.1, 28.2 23.3, 23.3 19.3, 19.3 16.0, 16.0 69.6, 69.7 21.3, 21.3 6.50, 6.46 2.00, 1.96

assembly TCPBP···[Pd32+]4 TCPBP···[Pd32+]3 TCPBP···[Pd32+]2 TCPBP···[Pd32+] TCPBP···[Pd32+]4 TCPBP···[Pd32+]3 TCPBP···[Pd32+]2 TCPBP···[Pd32+]

%: K14, %V 36.7, 35.8 25.4, 25.2 17.5, 17.7 12.1, 12.5 79.0, 78.1 16.6, 17.1 3.50, 3.75 0.75, 0.82

the dyes are associated with at least one cluster, and the major component is the dye···[Pd32+]4 assembly. Transient Absorption Spectroscopy. This technique was employed to extract the rates for excited-state quenching of the dyes by [Pd32+] because the assemblies are not luminescent. H

DOI: 10.1021/acs.inorgchem.5b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 10. Top: Time evolution of the transient spectra of TCPP in MeOH at 298 K. Middle: Spectra of transient species deconvoluted from the experimental spectra above. Bottom: Rise times and decays measured at various wavelengths [λexc = 600 (left) and 400 nm (right)].

Figure 11. Top: Time evolution of the transient spectra of the TCPP···[Pd32+]x assemblies in MeOH at 298 K (λexc = 600 nm). Middle: Spectra of transient species deconvoluted from the experimental spectra above. Bottom: Rise times and decays measured at various wavelengths. The [CO2−]/[Pd32+] stoichiometries are 1:1 (left) and 1:2 (right). See Table 7 for the relative proportions.

of the transient absorption spectra changes drastically. Instead of a strong single signal associated with the T1 species (see Figure 10, middle left; λexc = 600 nm) decaying in the nanosecond time scale, two bleached peaks in the Soret region are depicted and are decaying with different kinetics (Figure 11, top). Deconvolution of these time-resolved experimental spectra leads to four species (Figure 11, middle), one of which is the signature of the T1 component. It is worth noting that the intensity of this T1 signal decreases as the [CO2−]/ [Pd32+] ratio increases, in agreement with the data of Table 7 (the relative amount of free TCPP is respectively 13.3 and 0.6% for the 1:1 and 1:2 stochiometries). The three other components exhibit spectral shapes that strongly differ from that of TCPP alone (Figure 10). These species are the chargeseparated states (dye•+···[Pd3•+]), as previously demonstrated.8 Upon an increase in the [CO2−]/[Pd32+] ratio, the deconvoluted transient signals exhibit approximately the same shape but their relaxation times decrease. This phenomenon, also observed for the TCPBP analogues below, is related to an external heavy-atom effect driven by simple ion pairing (different from host−guest interactions) between the free dye and clusters. Indeed, the nanosecond component for free TCPBP (Figure 12, middle right, red line) decays more slowly than that for the TCPBP···[Pd32+]x assemblies with a 1:2 [CO2−]/[Pd32+] ratio (Figure 12, bottom). The free TCPBP also exhibits a dominant T1 signal. Again, its lifetime cannot be measured accurately because of the delay line (λexc = 680 nm excitation in the S1 manifold). The weak signal (pale green, τ = 361 ps) is likely the S1 species. The medium-intensity signal (blue line decaying at 0.28 ps) could be due to the solventinduced vibrationally relaxed S1 species, which disappears upon the addition of [Pd32+]. Upon generation of the TCPBP···[Pd32+]x assemblies, the T1 (yellow, ∼16 ns) and S1 (green, ∼30 ps) species remain as weak species exhibiting shorter kinetics, but three new

Figure 12. Top: Time evolution of the transient spectra of the TCPBP···[Pd32+]x assemblies in MeOH at 298 K (λexc = 680 nm). Middle: Spectra of transient species deconvoluted from the experimental spectra above. Bottom: Tise times and decays measured at various wavelengths. The [CO2−]/[Pd32+] stoichiometries are 1:0 (left) and 1:2 (right). See Table 7 for the relative proportions.

transients appear (Figure 12, middle right; purple, red, and turquoise lines). Time evolution of the transient spectra also shows a broader bleached signal in the vicinity of the Soret band. Deconvolution of new three transient species is I

DOI: 10.1021/acs.inorgchem.5b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry *E-mail: [email protected].

reminiscent of what was observed for the TCPP analogues in Figure 11. For the TCPBP···[Pd32+]x assemblies, the decays are respectively 0.33, 5.16, and 105 ps. For the TCPP···[Pd32+]x ones, these decays are 0.82, 7.05, and 47 ps for the same [CO2−]/[Pd32+] ratio. We assign these transients to chargeseparated states as well (dye•+···[Pd3•+]). However, it is not possible to assign which dye···[Pd32+]x assembly is which. However, electrostatic arguments would intuitively suggest that, as the number of [Pd32+] dication species around the dye•+ increases, the total positive charge increases around this radical cation, and consequently this central monocation unit should be easier to reduce. Therefore, back electron transfer (dye•+··· [Pd3•+] → dye···[Pd32+]) should be faster as x increases in the dye···[Pd32+]x assemblies. The similarities in the time scale for these processes (0.82, 7.05, and 47 ps for TCPP···[Pd32+]x and 0.33, 5.16, and 105 ps for TCPBP···[Pd32+]x) suggest that the driving forces are similar, which is consistent with the similarity in the photoinduced (forward) electron transfer (deduced from the CV traces in Figure 2 and Table 2). These time scales are also similar to those reported for MCP (1.0 and 65.5 ps) and DCP (1.2, 36.3, and 80.7 ps; structures shown in Chart 1).8 Moreover, attempts to observe the rise times of these chargeseparated species stubbornly failed in all cases. The rises for each transient signal occur well within the excitation pulse (fwhm = 85 fs). This means that photoinduced electron transfer (dye*···[Pd32+] → dye•+···[Pd3•+]) occurs at a time scale of