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Metal Dependence on the Bidirectionality and Reversibility of the Singlet Energy Transfer in Artificial Special Pair-Containing Dyads Adam Langlois,† Jean-Michel Camus,‡ Paul-Ludovic Karsenti,† Roger Guilard,*,‡ and Pierre D. Harvey*,† †

Département de Chimie, Université de Sherbrooke, Sherbrooke J1K 2R1, PQ, Canada Institut de Chimie Moléculaire de l’Université de Bourgogne, UMR CNRS 6302,Univ. Bourgogne Franche-Comté, 9 Avenue Alain Savary, BP 47870-21078 Dijon, France

‡

S Supporting Information *

ABSTRACT: The demetalation of a precursor dyad, 3, built upon a zinc(II)-containing artificial special pair and free-base antenna, leads to a new dyad, 4, for singlet energy transfer composed of cofacial free-base porphyrins (acceptor), [Fb]2 bridged by a 1,4-C6H4 group to a free-base antenna (donor), [Fb]. This dyad exhibits the general structure [M]2-C6H4-[Fb], where [M]2 = [Fb]2, and completes a series reported earlier, where [M]2 = [Mg]2 (2) and [Zn]2 (3). The latter dyads exhibit a bidirectional energy-transfer process at 298 K for 2 and at 77 K for 3. Interestingly, a very scarce case of cycling process is observed for the zinc-containing dyad at 298 K. The newly reported compound 4 exhibits a quasi unidirectional process [Fb]*→[Fb]2 (major, kET = 2 × 1011 s−1 at 298 K), where the remaining is [Fb]2*→[Fb] (minor, kET = 8 × 109 s−1 at 298 K), thus completing all possibilities. The results are analyzed in terms of molecular orbital couplings (density functional theory computations), Förster resonance energy transfer parameters, and temperature dependence of the decay traces. This study brings major insights about artificial special pair-containing dyads and clearly contributes to a better understanding of the communication between the two main components of our models and those already described in the literature.

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INTRODUCTION Cofacial and slip cofacial diporphyrins and related compounds represent a particular class of dimeric species, where interplanar cooperativity leads to different chemical and physical properties compared to their corresponding monoporphyrins and analogues,1 including electron transfer2 and energy transfer.3 When the distance between the two macrocycles is short the face-to-face dimer behaves as a single entity and represents a good model for the special pairs encountered in the photosynthetic membranes of plants and photosynthetic bacteria.4 Our group has reported two examples for models where a slipped cofacial bisporphyrin dimer is placed in the central core of anchored porphyrin units in a dendrimeric geometry to mimic the antenna effect in the photosynthetic membranes in plants.5 The simplest structure to investigate the mechanism for singlet energy transfer consists of a cofacial porphyrin dimer flanked by a single monoporphyrin, whether they are covalently held by a bridge or not.6 The most convenient spacer offering a short interplanar distance is the 1,8-biphenylene commonly called DPB.7 This species induces ππ interactions easily deduced by UV−vis and emission spectroscopy, where the porphyrin absorption and fluorescence bands become notoriously large, thus providing a clear diagnostic signature for this bisporphyrin chromophore.8 This property leads to a curious and yet © XXXX American Chemical Society

interesting situation, where the J-integral (i.e., parameter quantifying the spectral overlap between the emission band of the energy donor with the absorption band of the acceptor and controlling the rate for energy transfer according to the dipole− dipole Fö rster theory)9 is significant in both directions (i.e., donor*→acceptor, and acceptor*→donor). This bidirectionality has been encountered in Nature, where the energy transfers between the special pair (slipped cofacial chlorophyll dimer) and its antenna inside the photosynthetic membrane proceeds in equilibrium. Indeed, a special pair*↔antenna* energy transfer equilibrium was reported for photosystem II (special pair P680*↔antenna C670* occurring at ∼0.1 ps).10 Similarly, two energy transfer processes were reported for the purple photosynthetic bacteria using selective excitations (k1, antenna B875*→special pair P865, at 25 ps, and 8 ps for k−1, special pair P865*→antenna B875).11 These processes were demonstrated to be a key regulation feature for the subsequent oxidation of the special pair (i.e., charge-separation step), thus securing stability.12 We recently started a mechanistic investigation on a series of dyads composed of a cofacial slipped bisporphyrin dimer held by Received: November 7, 2016

A

DOI: 10.1021/acs.inorgchem.6b02684 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Structures of dyads 1−4. Ar = 3,5-di-t-butylphenyl, R = n-nonyl.

Figure 2. Three possible scenarios for energy transfer mechanism in [M]2-C6H4-[Fb] dyads. Frames A, B, and C correspond to the situation, respectively, for dyads 4 (this work), 2, and 3 (at room temperature).

and reassign the mechanism more clearly to a cycling one (Figure 2C). The conclusion appears to be that the presence of cycling is metal-dependent at 298 K, more specifically for [Zn]2.

a DPB spacer, acting as an artificial special pair ([M]2), and covalently linked to a monoporphyrin, acting as the antenna ([Fb]; dyads 1−3; Figure 1).13 Three rather different singlet− singlet energy transfer behaviors were depicted when examining the different dyads (Figure 2). The bridgeless dyad 1, [Zn]2−[Fb], was reported to exhibit a unidirectional process with no cycling [Fb]*→[Zn]2 with a rate of kET = ∼5 (ps)−1 at 298 K (rise time) taking advantage of the convenient presence of stimulated fluorescence in the femtosecond transient absorption spectra (TAS).13a Dyad 2, [Mg]2C6H4-[Fb], exhibits a bidirectional behavior (Figure 2B) at 298 K with no evidence of cycling clearly depicted from the presence of a combination of four kinetic components extracted from the fluorescence decays and femtosecond TAS (rise time) described as quenched and unquenched S1 signal of [Mg]2 and [Fb].13c Finally, dyad 3 exhibits a bidirectional process with no cycling at 77 K (Figure 2B) but shows obvious cycling at 298 K (Figure 2C) deduced by the presence of only two components showing decay kinetics with intermediate values located between those for efficiently quenched and unquenched S1 signals.13b Moreover, the latter observations suggest that there is likely an activation energy consideration. In a search for the parameters that control the mechanisms depicted in Figure 2 (A, B, or C), we now report the synthesis and photophysical investigation of dyad 4, which proceeds via a (quasi) unidirectional mechanism (Figure 2A). Unexpectedly, it exhibits the fastest rate of the three structurally related series dyads 2−4 and equals that for dyad 1 (also kET = ∼5 (ps)−1). This result urged us to reconsider the data for dyad 1

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EXPERIMENTAL SECTION

Materials. Zinc(II) 5-{8-[zinc(II)-10,20-dinonylporphyrin-5-yl]biphenylene-1-yl}-15-[4-{10,20-bis-3,5-di-tert-butyl-phenyl)porphyrin5-yl}phenyl]-10,20-dinonylporphyrin (dyad 3),13b the model compounds 5-{8-[10,20-dinonylporphyrin-5-yl]-biphenylene-1-yl}-15(4,4,5,5-tetramethyl-[1,3,2]-dioxaborolan-2-yl)-10,20-dinonylporphyrin (model 5) 13c and 5-(4-bromophenyl)-10,20-bis(3,5-di-tertbutylphenyl)porphyrin (model 6)13c were prepared according a known procedure. 5-{8-[10,20-Dinonylporphyrin-5-yl]-biphenylene-1-yl}-15-[4{10,20-bis-3,5-di-tert-butylphenyl)porphyrin-5-yl}phenyl]-10,20-dinonylporphyrin (dyad 4). HCl aqueous solution (10 mL, 4 M) was added onto a solution of zinc(II) 5-{8-[zinc(II)-10,20-dinonylporphyrin-5-yl]-biphenylene-1-yl}-15-[4-{10,20-bis-3,5-di-tert-butylphenyl)-porphyrin-5-yl}phenyl]-10,20-dinonylporphyrin, dyad 3 (24 mg, 0.011 mmol) in 10 mL of CH2Cl2. The reaction mixture was vigorously stirred for 30 min. The organic layer was separated, washed with a saturated NaHCO3 aqueous solution (2 × 20 mL), and dried over MgSO4. After evaporation until dryness, the residue was recrystallized in CH2Cl2/CH3OH affording a purple microcrystalline powder (21 mg, 94%). 1H NMR (300.16 MHz, CD2Cl2): δ 10.33 (s, 1H, meso), 9.76 (d, 1H, J = 6.8 Hz, C6H4), 9.56 (d, 2H, J = 4.8 Hz, β), 9.47 (d, 2H, J = 4.6 Hz, β), 9.29 (d, 2H, J = 4.3 Hz, β), 9.25 (s, 1H, meso), 9.19 (d, 2H, J = 4.6 Hz, β), 9.13 (d, 2H, J = 6.7 Hz, C6H4), 8.93 (d, 2H, J = 4.3 Hz, β), 8.87 (d, 2H, J = 4.6 Hz, β), 8.82 (d, 2H, J = 4.4 Hz, β), 8.70 (d, 2H, J = 3.6 Hz, β), 8.57 (m, 6H, β), 8.48 (d, 2H, J = 7.5 Hz, C6H4), 8.34 (s, 2H, C6H3), 8.27 (m, 4H, 2 β + 2C6H3), 7.98 (t, 2H, J = 1.8 Hz, C6H3), 7.89 B


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Inorganic Chemistry

Fast Kinetic Emission Decay Measurements. The laser source was the same as described above. The instrument response function increased to a half width at half-maximum of 8 ps after passing through the optics at 298 K and become 12 ps after passing through a Dewar for 77 K measurements. The detector was a Streak Camera (Axis-TRS, Axis Photonique Inc.) limited to a maximum of ∼2.5 ns. The results were globally analyzed with the program Glotaran (http://glotaran.org) permitting to extract a sum of independent exponentials (I(λ, t) = C1(λ) × exp(−t/τ1) + C2(λ) × exp(−t/τ2) + ...).

(d, 1H, J = 7.9 Hz, C6H4), 7.47 (d, 1H, J = 8.0 Hz, DPB), 7.35 (d, 1H, J = 6.6 Hz, DPB), 7.28 (m, 2H, DPB), 7.10 (t, 1H, J = 8.2 Hz, DPB), 7.05 (t, 1H, J = 7.9 Hz, DPB), 3.87 (m, 4H, CH2), 3.77 (m, 2H, CH2), 3.65 (m, 2H, CH2), 2.02 (m, 8H, CH2), 1.70 (s, 18H, tBu), 1.63 (m, 26H, tBu + CH2), 1.49−1.21 (m, 40H, CH2), 0.91 (m, 12H, CH3), −2.73 (s, 2H, NH) and −6.59 (s, 4H, NH) ppm. Electrospray ionization highresolution mass spectrometry (ESI HRMS): m/z 1039.6337 calcd for C142H160N12Na2, 1039.6347 (M + 2Na)2+. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS): m/z 2033.3 calcd for C142H161N12, 2034.1 (M + H+). UV−vis (tetrahydrofuran (THF)) λmax (log ε) = 400 (5.49), 415 (5.72), 511 (4.45), 542 (4.19), 584 (4.00), 641 (3.66), 665 (3.67) nm. Instruments. NMR spectra for the reported compounds were recorded at room temperature using Bruker Avance 300 instrument with the chemical shifts reported as δ in parts per million. Accurate mass measurements (ESI HRMS) were performed using a Bruker microTOFQTM ESI-TOF mass spectrometer. MALDI-TOF mass spectrometry was performed with a Bruker Ultraflex II MALDI-TOF mass spectrometer using dithranol as the matrix. All samples were freshly prepared and measured within 1 h. Absorption spectra were measured on a Varian Cary 300 Bio UV−vis spectrometer at 298 K and on a Hewlett-Packard 8452A diode array spectrometer with a 0.1 s integration time at 77 K. Steady-state UV−vis emission spectra were acquired on an Edinburgh Instruments FLS980 phosphorimeter equipped with single monochromators. All emission spectra were corrected for instrument response. Fluorescence lifetime measurements were made with the FLS980 phosphorimeter using a 378 nm ps pulsed diode laser (full width at half-maximum (fwhm) = 90 ps) as an excitation source. Data collection on the FLS980 system is done by time-correlated singlephoton counting (TCSPC) system. Density Functional Theory Calculations. All density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed with Gaussian 0914 at the Université de Sherbrooke with the Mammouth supercomputer supported by Le Réseau Québécois De Calculs Hautes Performances. The DFT geometry optimizations as well as TDDFT calculations were performed using the B3LYP method.15−24 A 6-31g* basis set was used for all carbon, nitrogen, and hydrogen located on the biphenylene and benzene spacers as well as all three porphyrin macrocycles, while a 3-21g* basis set was applied to the solubilizing groups.25−30 A THF solvent field using the polarizable continuum model (PCM) and the integral equation formalism (IEFPCM) was applied to all calculations.31 The calculated absorption spectra were obtained from GaussSum 2.1.32 Femtosecond Transient Absorption Spectroscopy. The femtosecond transient spectra and decay profiles were acquired on a homemade system using the second-harmonic generation of a Soltice (Spectra Physics) Ti-sapphire laser (λexc = 398 nm; fwhm = 75 fs; pulse energy = 0.1 μJ per pulse, repetition rate = 1 kHz; spot size ∼500 μm), a white light continuum generated inside a sapphire window and a custom-made dual CCD camera of 64 × 1024 pixels sensitive between 200 and 1100 nm (S7030, Spectronic Devices). The delay line permitted to probe up to 4 ns with an accuracy of ∼4 fs. The results were analyzed with the program Glotaran (http://glotaran.org) permitting to extract a sum of independent exponentials (I(λ , t ) = C1(λ) × e−t1/ τ + C2(λ) × e−t2 / τ + ···) that fits the whole three-dimensional (3D) transient map.

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RESULTS AND DISCUSSION Synthesis. The demetalation of dyad 3 with acid leads to dyad 4 cleanly, which was characterized by 1H NMR, UV−vis, and mass spectrometry (Scheme 1). The replacement of Zn by protons inside the porphyrin macrocycles is readily detected by 1H NMR. Indeed, two signals are depicted at −2.73 and −6.59 ppm integrating for two and four H, respectively. The more negative chemical shift for the latter signal is explained by the overlap of the anisotropic cones formed by the cofacial aromatic rings creating a stronger magnetic field than what would be seen in the former. The absence of zinc is readily confirmed by the disappearance of the heavier ion peak of dyad 3 in the mass spectra. The UV−vis spectra exhibit two Q-bands at 641 and 665 nm indicating the presence of two different types of free-base porphyrin. Luminescence Spectra Analysis. The absorption, excitation, and fluorescence spectra and data of models 5 and 6 and dyad 4 are compared in Figure 3 and Table 1. The fluorescence spectra of 4 and 5 are notoriously broad at both 77 and 298 K, which is a characteristic signature of the closely spaced DPB-linked bisporphyrin chromophores.8a This broadness is due to the incorporation of new low-frequency vibronic peaks associated with the tight cofacial structure. The resemblance of the fluorescence spectra indicates unambiguously that the emitting fragment in 4 is the cofacial unit [Fb]2. The broad luminescence signal strikingly differs from that observed for compound 6, which exhibits sharp fluorescence peaks at 645 and 712 nm at 298 (fluorescence quantum yield, ΦF = 0.019 in 2MeTHF, spectrum shown in Figure 3 as an example) and 77 K.13b These sharp peaks are clearly absent (or quasi-absent) from the fluorescence spectra of dyad 4 indicating efficient quenching. The good superposition of the excitation spectra with the absorption spectra unambiguously indicates the presence of efficient singlet−singlet energy transfer of the type [Fb]*→[Fb]2 (Figure 3). Computations. The interpretation of the electronic structure, absorption spectra, nature of the lowest excited states, and presence of molecular orbital (MO) coupling (or not) was addressed by DFT and TDDFT computations. First, the geometry was optimized (Figure 4; top), and relevant structural data were

Scheme 1. Synthesisa of Dyad 4

a

Ar = 3,5-di-tert-butylphenyl, R = n-nonyl. C


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Inorganic Chemistry

Figure 3. Steady-state absorption, emission, and excitation spectra for models 6 (top) and 5 (middle) and dyad 4 (bottom) in 2MeTHF at 77 K (right) and 298 K (left). Structures of models 5 and 6 and dyad 4 are provided on the right Ar = 3,5-di-tert-butylphenyl, R = n-nonyl.

within the [Fb]2 chromophore (Table S1). This computed relative position of the lowest energy transition corroborates that the lowest energy singlet excited state is indeed located within the artificial special pair and assigns the role of this chromophore being the energy acceptor. Transition No. 3, at higher energy, is composed of transitions whose calculated atomic contributions are located in the [Fb] unit (H-5→L+1 (15%), H-2→LUMO (29%)), thus corroborating that [Fb] has the role of the energy donor. Interestingly, this upper state is mixed with a transition located within the [Fb]2 residue (HOMO→L+3 (17%)). This computed outcome suggests the presence of weak MO couplings. This coupling is also shown in the minor atomic contributions located on the other free porphyrin ring for each MO (Table S1), with the slight exception of HOMO−1. Moreover, the amplitude of couplings is approximately the same for all three dyads 2−4 (Tables S2 and S3).13b,c The DFT calculations allow us to confirm that the [Fb]2 is the low-energy chromophore (energy acceptor) and that these excited states exhibit evidence for weak MO couplings between the two chromophores, which is related to the fact that significant MO coupling contributes to an accelerated energy transfer rate.6a,13 Fluorescence Lifetimes. The comparison of the fluorescence lifetimes τF of 4, 5, and 6 at 298 and 77 K is provided in Table 3. The fact that the τF(5) values are shorter than τF(6) values is consistent with the known increase of the nonradiative rate constants (k nr ) going from the [Fb] to the [Fb] 2 chromophore, an effect associated with additional motions specific to slipped dimers.8a Moreover, the τF(5) values compare more favorably to those extracted for dyad 4 in comparison with the τF(6) ones. These two combined observations corroborate well the conclusion drawn from the spectra that the emitting chromophore in dyad 4 is unambiguously [Fb]2. The decays of the [Fb]2 residue at 685 nm also exhibit a weak intensity component of several percents in the time scale of a hundred of

Table 1. Absorption and Fluorescence Data for the Models 6 and 5 and Dyad 4 T (K)

absorption data (nm) Soret

613b 513c

4

298 77 298 77

415 418 401 402

298

400(sh), 415 404, 420

77

fluorescence data (nm)

Q-bands 513, 549, 590, 644 508, 544, 580, 637 522, 557, 600, 657 520, 560, 596, 652

645, 712 638, 705 688, 721, 756 (sh) 657, 691,716, 728, 763(sh) 511, 542, 586 641, 665 687, 749 518, 550, 586,604(sh) 648

660, 735

extracted. The HOMO−4, HOMO−3, and LUMO+2 to LUMO+5 (HOMO = highest occupied molecular orbital; LUMO = lowest unoccupied molecular orbital) simultaneously exhibit atomic contributions from both free-base porphyrins on the [Fb]2 fragment, consistent with the singularity of this chromophore. Somewhat surprisingly, the HOMO−1 and HOMO are quasi-degenerate with π-systems individually located on each porphyrin of [Fb]2. The MOs HOMO−2, LUMO, and LUMO+1 exhibit atomic contributions of the π-systems located on the [Fb] unit. The relative atomic contributions for the frontier MOs are presented in Table S1 in the Supporting Information. By applying an arbitrary thickness (fwhm) of 500 cm−1 to each of the calculated electronic transitions (TDDFT), a simulated spectrum is generated and compares favorably to the experimental spectrum (Figure S2), except for a slight blue shift of the calculated positions of the Q-bands. The calculated position of the lowest energy transitions (Nos. 1 and 2) are found to be mainly composed of HOMO→LUMO +2 and HOMO−1→LUMO+2 (Table 2). The calculated atomic contributions indicate that these transitions are located D


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Figure 4. Optimized geometry (top) of dyad 4 showing the center-to-center axis (13.16 Å in purple), and representation of the frontier MOs (bottom; in eV; H = HOMO, L = LUMO.

Table 2. Calculateda Positions and Oscillator Strengths (F) of the 3 First Electronic Transitions for Dyad 4

a

No.

λ (nm)

F

major contributions (%)

1 2 3

589.8 580.4 571.6

0.0547 0.0189 0.0307

HOMO→L+2 (49) H-1→L+2 (51) H-5→L+1 (15), H-2→LUMO (29), HOMO→L+3 (17)

See Table S5 for the 100 first calculated transitions.

Table 3. Singlet Excited-State Lifetimes of 4, 5 and 6 in 2MeTHF at 298 and 77 K λFluo (nm)

τF (TCSPC unless stated otherwise)a,b

comp

chrom

dyad 4

[Fb]2

685

686

model 5 model 6

[Fb] [Fb]2 [Fb]

∼645 688 640

∼640d 655 640

298 K

77 K

298 K

77 K

9.8 ± 2.1 ns (major)uq + a minor component (χ2 = 1.031) ∼121 ps (Streak)q ≤8 ps (Streak)d 5.0 ± 0.1 ps (TAS)c 8.4 ± 0.5 ns (χ2 = 1.076) 13.9 ± 0.1 nse (ΦF = 0.019)e

13.0 ± 0.5 ns (major)uq + a minor component (χ2 = 1.069) ∼241 ps (Streak)q ∼8.5 ps (Streak)d not accessible (TAS)c 13.1 ± 0.5 ns (χ2 = 1.076) ±0.2 nse

a

The linear and log scale decay traces are placed in Figures S3−S6 and were analyzed using Exponential Series Method. bq and uq = quenched and unquenched fluorescence, respectively. For model 5, no short picosecond component was observed. cFemtosecond TAS (λexc = 510 nm, power =25 μW (0.05 μJ/pulse) at 500 Hz). dThis energy donor chromophore, [Fb], is weakly emissive and TAS experiments are not possible at 77 K. eLifetime and quantum yield values taken from ref 13b.

energy transfer. The estimated τF value of 7.0 ps (298 K) cannot be trusted, as it falls under the IRF (8 ps). Conversely, the 8.5 ps datum (77 K) may be used as an approximation. The two other signals are the [Fb]2 fluorescence as determined by steady-state measurements (Figure 3). A little distortion is detected at 298 K (turquoise trace) due to the weakness of the signal and consequently impacting on the accuracy to define the resulting deconvoluted spectrum. At 77 K, the two components are identical. Two τF values are depicted as predicted by TCSPC. The τF values exceeding 2 ns are not accurate due to the limit of the instrument. This is of a minor consequence, as these are

picoseconds at both temperatures based on TCSPC measurements. The time base (the pulse width is ∼90 ps) and low intensity of this component precluded its accurate measurements by TCSPC, as such a Streak camera with higher temporal resolution and sensitivity was used to attempt to extract the minor component (Figure 6). At best fit, the time-resolved fluorescence spectra at 298 and 77 K were deconvoluted into three components. The weakest signal (black) exhibits a maximum at ∼640 nm (Figure 5 bottom, see arrow), which is characteristic of the donor fluorescence [Fb]. The negative portion of this signal is a clear indication of an E


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Inorganic Chemistry

Figure 5. (top) Time-resolved spectra of dyad 4 in 2MeTHF at 298 K (left) and 77 K (right). (bottom) Three deconvoluted components showing the [Fb] (black; τF ≤ ∼8 ps) and [Fb]2 (green; τF > 2 ns; turquoise; ∼125 ps) fluorescence. The shape of the turquoise trace is difficult to define accurately due to its relative weakness.

Table 4. Comparison of the Energy Transfer Properties for Dyads 1−9 at 298 K chromophore dyad

a

donor

acceptor

3

[Fb]

[Zn]2

2

[Fb]

[Mg]2

4

[Fb]

[Fb]2

1 7 8 9

[Fb] [Zn] [Zn] [Zn]

[Zn]2 [Fb] [Fb] [Fb]

kET (s−1)

τ(S1) (ps) 298 K 440 2640 40 210 5.0 (major) 125 (minor) 5.1 0.55 3.4 10

process

298 K

ET typea

refa

[Fb]*→[Zn]2 [Zn]2*→[Fb] [Fb]*→[Mg]2 [Mg]2*→[Fb] [Fb]*→[Fb]2 [Fb]2*→[Fb] [Fb]*→[Zn]2 [Zn]*→[Fb] [Zn]*→[Fb] [Zn]*→[Fb]

1.8 × 10 0.31 × 109 2.5 × 1010 0.46 × 1010 2.0 × 1011 ∼0.8 × 1010 2.0 × 1011 1.8 × 1012 2.9 × 1011 1.0 × 1011

cycling

13b

bidir

13c

∼unidir

tw

unidir unidir unidir unidir

13a 34 33 33

9

tw = this work, bidir = bidirectional, unidir = unidirectional.

compares favorably to that reported for dyad 2 (40 ps, 298 K).13c It also compares well with other dyads with the general structures [Zn]-C6H4-[Fb] exhibiting unidirectional energy transfer processes (see dyads 8 (3.4 ps) and 9 (10 ps) in Figure 8).33 Furthermore, TAS measurements did not show any evidence for the formation of radical ions that would be associated with charge separation and electron transfer between the cofacial pair and the antenna. The time scale for the energy transfer (0.55 ps) for a directly bonded dyad [Zn]−[Fb] (7) has also been reported and is used for comparison purposes (Figure 7, Table 4).34 Moreover, the previously reported longer 40 ps value for dyad 2 ([Fb]*→[Mg]2) is also suggestive that the fluorescence of the [Fb] pendent group may be visible. This was indeed the case where a weak shoulder at 645 nm was unambiguously recorded. Using kET(S1) = (1/τF(donor)) − (1/τF(donor)°), a value of 2.0 × 1011 s−1 is evaluated for dyad 4 at 298 K. The comparison of the two extracted kET values in 4 (i.e., [Fb]*→[Fb]2; major, 2 × 1011 s−1) and [Fb]2*→[Fb] (minor, ∼8 × 109 s−1)) indicates ∼2 orders of magnitude faster for the former direction. This difference (major/minor ratio) suggests a mostly unidirectional pathway for dyad 4. Noteworthy, this trend is also noted at 77 K (kET ≈ 1.2 × 1011 and 4 × 109 s−1 for [Fb]*→[Fb]2 and [Fb]2*→[Fb], respectively. The Förster Theory. The kET values can be calculated using eq 1, where kF° is the radiative rate constant of the donor in the

already accurately measured by TCSPC (9.8 and 13.0 ns at 298 and 77 K, respectively; Table 3). These more intense components are due to the unquenched [Fb]2 chromophore. Concurrently, the weaker and shorter-lived component (∼121 (298) and ∼241 ps (77 K)) are the quenched fluorescence of this same chromophore and reflect the presence of weak energy transfer in the [Fb]2*→[Fb] direction. The kET(S1) values are extracted from kET(S1) = (1/τF(donor)) − (1/τF(donor)°),3b where τF(donor) and τF(donor)° are the fluorescence lifetimes of the donor, respectively, in the presence and absence of an energy acceptor. For the major process, [Fb]*→[Fb]2, the τF(6) values are used for τF(donor)°, but the τF(donor) at 298 K is not accessible, that is, τF ≤ 8 ps (Streak camera), and TAS is used to extract this value (below). At 77 K, τF(donor) ∼8.5 ps and τF(donor)° = 16.7 ns (Table 4), so kET (77 K) ∼1.2 × 1011 s−1. For the minor process, [Fb]2*→[Fb], τF(donor) ∼121 and 241 ps and τF(donor)° = 9.8 and 13.0 ns at 298 and 77 K, respectively. The corresponding kET values are, respectively, 8 × 109 and 4 × 109 s−1. TAS Experiments. The monitoring of the TAS signals in the Soret region of dyad 4 is shown in Figure 6. A clear rise time of 5 ps is observed for the peak at 401 nm, which is accompanied by a decay of the signal at 420 nm with the same kinetics. This rise time is clearly associated with a very fast energy transfer process, and the time scale is fully consistent with the quasi-absence of fluorescence of the [Fb] chromophore in dyad 4 (Figure 3) and F


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Inorganic Chemistry

Figure 6. fs-TAS of 4 in 2MeTHF at 298 K (λexc = 510 nm, power = 25 μW (0.05 μJ/pulse) at 500 Hz, pulse width = 100 fs). (top left) A 3D map of the TAS signal showing the rise of the [Fb]2 (401 nm) unit and decay of [Fb] (420 nm). (top right) Kinetic traces of the 401 and 420 nm signals. (bottom left) Spectral signatures of the transient species extracted by global analysis showing three components corresponding to the singlet states of [Fb] (red), [Fb]2 (green), and a long-lived triplet state of [Fb] (black; note that the delay line is limited to ∼8 ns, so its triplet lifetime is not accessible). (bottom right) Time slices showing the rise and decay of [Fb]2 and [Fb], respectively, with delay times between the pump and probe pulses.

Figure 7. Comparison of the time scales reported for the singlet energy transfer at 298 K in the structurally related dyads 1,13a 3 (cycling),13b 7,34 8, and 9.33 Note that dyad 3 becomes bidirectional at 77 K with τF < 100 ps values (both directions) with no cycling.13b 2⎞ ⎛ o −25 kF (D)κ ⎟·J ⎜ kForster 8.785 10 ≅ × · ̈ ⎝ r 6n 4 ⎠

absence of an acceptor, n is the refractive index of the solvent, r is the center-to-center distance separating the donor and acceptor, κ2, an orientation factor, ranging between 0 and 4, that describes the alignment of the transition dipole moments of the donor and acceptor, and J (also called J-integral) is the spectral overlap between the donor emission and the acceptor absorption (as calculated by eq 2), where FD is the emission spectrum of the donor, εA is the absorption spectrum of the acceptor in units of absorptivity, and FD* is the normalized emission spectrum of the donor (such that the area under the curve is unity).

(1)

∞

J(M =

−1

cm

∫0

∞

−1

4

nm ) =

∫0 FD(λ)εA (λ)λ 4 dλ

FD*(λ)εA (λ)λ 4 dλ

∞

∫0 FD(λ)dλ (2)

For dyad 4, the comparison of the spectral overlaps for a bidirectional mechanism is presented in Figure 8. Visually, the G


Article

Inorganic Chemistry

Figure 8. Overlay in a normalized form of the absorption spectrum of model 5 with the fluorescence one of model 6 (left) and the absorption spectrum of model 6 with the fluorescence one of model 5 (right) thus representing the [Fb]*→[Fb]2 (J = 2.44 × 1014 M−1 cm−1 nm4) and [Fb]2*→[Fb] (J = 0.20 × 1014 M−1 cm−1 nm4) processes, respectively.

the calculated kFörster and experimental values shows a larger value for the former (by 36-fold) unambiguously indicating that the energy transfer mechanism is FRET-dominated. This conclusion is drawn because, if the Dexter mechanism was dominant, then experimental energy transfer rate should be larger than the calculated kFörster one. Finally, the Dexter mechanism requires a good orbital overlap between the donor and acceptor, for which no evidence was found in the DFT calculations. Instead, the current calculations show weak degree of atomic contributions calculated on the other chromophore; that is, weak MO coupling means weak MO overlap. Although we cannot demonstrate that the energy transfer occurs purely through an FRET mechanism, we are confident that it is largely dominated by it and does not exclude the Dexter one, at least as a minor contribution.

gray region (i.e., the overlap) is larger for the [Fb]*→[Fb]2 process in comparison with that for [Fb]2*→[Fb]. More interestingly, the fluorescence spectrum of [Fb] is completely overlapping with the tail of the absorption spectrum of the acceptor [Fb]2 (Figure 8, left), whereas only a small portion of the [Fb]2 fluorescence band overlaps with the absorption spectrum of [Fb] (Figure 8, right). The values of the J-integrals are found to be 2.44 × 1014 for [Fb]*→[Fb]2 and 0.20 × 1014 M−1 cm−1 nm4 for [Fb]2*→[Fb]. These observations strongly support the unidirectional behavior of dyad 4. Because the structure and energy donor are identical for dyads 2−4, the only variable is the J-integral, which is dependent upon the metal. For dyad 2, bidirectional behavior was depicted.13c The J-integral values were not calculated, but the visual comparison of the overlaps is revealing. Visually, the portion of the fluorescence band of [Mg]2 overlapping with the absorption of [Fb] is clearly larger than that observed here for [Fb]2 and [Fb], suggesting that the [Mg]2*→[Fb] process is more favored than the [Fb]2*→[Fb] process. A similar situation is observed when we examine compound 3, which exhibits cycling. Intuitively energy transfer should occur in the [Fb]*→[M]2 direction, as [M]2 sits at lower energy; however, the reverse process is also observed due to the presence of a non-nil J-integral. When we then examine the three structures we see that when the cofacial pair contains metal ions we observe a stronger contribution of the reverse energy transfer process ([M]2*→[Fb]), but when the metal ions are removed the unidirectional energy transfer process is favored. This can be explained by the slight blue shift of the [M]2 emission when metal ions are placed in the porphyrin cycles. The ∼13 nm blue shift of the emissions of [Mg]2 and [Zn]2 (emission maxima at ∼675 nm)13b,c compared to that of [Fb]2 (emission maximum at 688 nm) allows for a larger spectral overlap with the free-base antenna leading to a larger J-integral for the ([M]2*→[Fb]) process when metal ions are used. Conclusively, the uni- and bidirectionality of dyads containing an artificial special pair is metaldependent by virtue of the J-integral. Using ΦD = 0.019 and τD = 13.9 ns (Table 3; i.e., kF° = 1.37 × 106 s−1), a center-to-center value (r) of 1.316 nm, and a J integral of 2.44 × 1014 M−1 cm−1 nm4 (for [Fb]*→[Fb]2) in eq 1, one obtains kFörster = (3.24 × 1012 × κ2) s−1. By using the average κ2 value of 2.22 (see Supporting Information for calculation),35 we obtain a theoretical kFö rster value of 7.19 × 10 12 s−1 (experimental = 2 × 1011 s−1). Such a difference between the calculated and observed values is not unusual, as the Förster theory is an approximation. In many cases, especially those that show covalent bonding exhibiting a π-system between a donor and acceptor, the Dexter energy transfer mechanism may play an important role. In this case, however, the comparison between

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DISCUSSION The similarity of the kET values for dyads 1 and 4 (5 ps) triggers serious questioning when taking into account the 1/r6 factor. Indeed, the r value is 8.41 Å in dyad 1,13a and assuming that the J-integrals are comparable for 1 and 4, then kFörster should be ∼50 times faster for dyad 1. Moreover, the comparison of the time scales (∼1/kET) for the singlet energy transfers for the bridgeless dyads 1 and 7 indicates a 10-fold difference between the two (kET (1) < kET (7)) despite the fact that the absorption and fluorescence bands of the [Zn]2 unit are significantly broader for 1 compared to 7 (i.e., the J-integral should be larger for the former dyad). The first clue comes from the comparison of the observed time scales of the singlet excited lifetimes of dyad 3 (cycling at 298 K) compared to those for dyads 8 and 9 (unidirectional; Figure 7). In the cycling case, dyad 3 exhibits quenched excited-state lifetimes of both [Zn]2 and [Fb] that notoriously are 2 orders of magnitude larger than those for 8 and 9 due to efficient “refilling” of these excited states upon cycling. The second clue comes from the difference in energy transfer time scales between 1 and 7. This difference is 1 order of magnitude also suggesting the presence of some cycling. However, this process clearly appears less pronounced than that observed for the 3 versus 8/9 series. This discrepancy is unambiguously related to their structural difference and therefore to their molecular motions. Indeed, the slipping motions of the cofacial [Zn]2 unit in 1 is clearly hindered by the directly bonded pendent [Fb] group compared to C6H4-[Fb]. The proof for this hypothesis arises from the temperature dependence of the energy transfer mechanism found in dyad 3 (i.e., respectively, cycling and not cycling at 298 and 77 K) and the obvious ring distortion of the [Zn]2 fragment observed in the optimized geometry of 1.13a H


Article

Inorganic Chemistry These two considerations lead to the conclusion that the activation energy to slipping motions in [M]2 differs depending upon the metal (here Zn, Mg, and 2H in dyads 2−4), where it is lower for Zn. Zinc-containing porphyrins are known to axially interact with ligand donors, including THF,36 so as the magnesium analogues but in a much lesser occurrence.37 An attempt was made to see whether the fluorescence decay traces and kinetic data for 4 in 2MeTHF (Tb = 78 °C), as presented in Table 3, turn out to be rigorously identical between 298 and 328 K. This result indicates that the barrier of activation must be larger than the thermal energy of the medium at 55 °C (i.e., kT = 2.73 kJ mol−1).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (P.D.H.) *E-mail: [email protected]. (R.G.) ORCID

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Roger Guilard: 0000-0001-7328-3695

CONCLUSION At 298 K, dyad 2 exhibits a bidirectional energy transfer process between the artificial special pair and antenna, whereas 3 presents a cycling system. These two metal-containing models mimic the energy processes observed in Nature, that is, the special pair*↔ antenna* equilibrium for photosystem II (special pair P680*↔ antenna C670* occurring at ∼0.1 ps)10 and the bidirectional process in the purple photosynthetic bacteria (antenna B875*→ special pair P865, at 25 ps, and 8 ps for special pair P865*→ antenna B875).11 It is still unclear whether equilibrium (i.e., cycling) should or not lead to a larger or smaller time scale for the overall delocalization of the absorbed energy by the dyad. The sampling is far too small to detect any reliable tendency. In this work, a metal-free system exhibits an unexpected quasiunidirectional process for [Fb]*→[Fb]2 (major process and 2 orders of magnitude faster), the minor process being [Fb]2*→[Fb] is unraveled despite the identical skeleton with dyads 2 and 3. This feature places this dyad in the same category of models 7−9 (Figure 7) but also at the frontier between bi- and unidirectional energy transfer dyads (Table 4). We have clearly confirmed the uni- and bidirectionnality of dyads containing an artificial pair is metal-dependent by virtue of the J-integral, whereby the presence of metal ions in the cofacial porphyrin system results in a blue shift of the bisporphyrin emission leading to a large J-integral and an increased rate of ([M]2*→[Fb]) energy transfer potentially allowing for the cycling of excitation energy. Therefore, this new model 4 (Frame A in Figure 2) completes the series of all possible energy transfer mechanisms when one of the components is a cofacial bismacrocycle and provides valuable information on why this is so. As an example it is of particular interest to note that the geometry of the model, that is, the presence or not of a spacer between the two main components of the model, is not a key parameter to direct the energy transfer, whereas the presence of metal ions appears to be. At this point, one may draw two hypotheses, (1) bidirectionality is often possible when considering the J-integral in both directions (Figure 8) but perhaps remains simply undetected due to the weakness of the second quenched signal as was the case here, and (2) the absence of cycling may be due to a larger activation barrier to the process. The current question is still what is the structural parameter that promotes cycling if the slipping motion in the cofacial special pair is not somehow involved considering that this contribution is obviously precluded in natural synthetic membranes? Further research is underway.

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Molecular orbital contributions for dyads 2, 3, and 4 as a function of molecular fragments, calculation of the orientation factor (κ2). First 100 calculated electronic transitions for dyad 4, and the linear and logarithmic decay traces of models 4 and 5 at 298 and 77 K. (PDF)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Natural Sciences and Engineering Research Council of Canada, the Fonds de recherche du Québec−Nature et technologies, the Centre Québécois des Matériaux Fonctionnels, and the Centre d’Études des Matériaux Optiques et Photoniques de l’Université de Sherbrooke. P.D.H. also thanks the Agence National de la Recherche for the grant of a Research Chair of Excellence and J.M.C. and R.G. thank the Centre National de la Recherche Scientifique.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02684. I


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