ultrafast singlet

ultrafast singlet

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Ultrafast Singlet-Singlet Energy Transfer in Self-Assembled via Metal-Ligand Axial Coordination of Free-Base Porphyrin-Zinc Phthalocyanine and Free-Base Porphyrin-Zinc Naphthalocyanine Dyads

Eranda Maligaspe,† Tatu Kumpulainen,‡ Helge Lemmetyinen,‡ Nikolai V. Tkachenko,*,‡ Navaneetha K. Subbaiyan,† Melvin E. Zandler,† and Francis D’Souza*,†

Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051, and Department of Chemistry and Bioengineering, Tampere UniVersity of Technology, P.O. Box 541, 33101 Tampere, Finland

ReceiVed: August 2, 2009; ReVised Manuscript ReceiVed: October 17, 2009

Singlet-singlet energy transfer in self-assembled via axial coordination of imidazole-appended (at different positions of one of the meso-phenyl entities) free-base tetraphenylporphyrin, H2PIm, to either zinc phthalocyanine, ZnPc, or zinc naphthalocyanine, ZnNc, dyads is investigated in noncoordinating solvents, o-dichlorobenzene and toluene, using both steady-state and time-resolved transient absorption techniques. The newly formed supramolecular dyads were fully characterized by spectroscopic, computational, and electrochemical methods. The binding constants measured from optical absorption spectral data were found to be in the rangeof 104-105 M-1 for the 1:1 dyads,suggestingfairlystablecomplexformation.Electrochemical and computational studies suggested that photoinduced electron transfer is a thermodynamically unfavorable processwhen free-baseporphyrinis excitedin these dyads. Selectiveexcitationof the donor free-baseporphyrin entity was possible in both types of dyads formed by either of the ZnPc or ZnNc energy acceptors. Efficient singlet-singlet energy transfer was observed in these dyads, and the position of imidazole linkage on the free-base porphyrin entity, although flexible, seems to have some control over the overall efficiency of excited energy transfer process. Kinetics of energy transfer was monitored by performing transient absorption measurements using both up-conversion and pump-probe techniques. Such studies revealed ultrafast singlet-singlet energy transfer in the studied dyads with time constants on the order of 2-25 ps depending upon the type of the dyad.

Introduction

The amazing features of Nature to tap available resources for the benefit of living organisms have intrigued mankind over centuries. One such classic example is photosynthesis which harvests solar energy and converts it to chemical energy in the form of transmembrane charge separation via a multistep electron transfer reaction.1 Chlorophylls and carotenoids are the primarypigmentsof naturallight harvestingsystemsresponsible for convertingthe energy of an absorbed photon into an electron excitation. The supramolecular organization of these pigments (antenna) allows the unidirectional excitation energy transfer toward a reaction center, the final destination of the collected energy.1,2 Inspired by this natural phenomenon,researchershave been attempting to mimic such complex processes with the help of synthetic molecular architectures, often termed as artificial photosynthesis.3-1 Research in this area holds promise not only to improve the fundamental understanding but also to advance technologically in building light energy harvesting photovoltaic devices, to construct molecular optoelectronics and to develop photocatalysts capable of producing hydrogen.12

In artificial photosynthesis, two photoinduced events are mainly targeted, viz., excitation energy transfer to mimic the antenna functionality, and electron transfer to mimic reaction center functionality.3-1 Several strategies have been employed to mimicthe naturalenergytransferprocess,includingcovalently linkeddyadsand polyads,13-16 polymers,17 dendrimers,18 and selfassembled systems.19 In these model compounds, successful excitationenergy transferfrom donor to the acceptorentitieshas been demonstrated.In the majorityof these studies,porphyrins20 and phthalocyanines21 have been used as a energy/electrondonor/ acceptor due to their close resemblance to the photosynthetic pigment,chlorophyll,andtheestablishedsyntheticmethodologies. Both macrocycles are ideal photoactive units with outstanding electronicproperties,namely,strongabsorptionin thevisibleregion andthepossibilityof fine-tuningtheredoxpotentials.Further,their absorptioncaneasilybe extendedintonear-IRregionby increasing macrocycleπ-conjugation.20,21

Self-assembly via metal-ligand axial coordination is one of the successful approaches developed to study photoinduced electron transfer in donor-acceptor dyads.1 However, utilization of this strategy to build dyads composed of different donor and acceptor fluorophores to mimic the natural energy transfer processhas not been fully explored.This has been accomplished in the presentstudy by constructingdonor-acceptordyads using free-base porphyrin as energy donor and zinc phthalocyanine as energy acceptor via axial ligand coordination.2 Further, zinc naphthalocyanine, a phthalocyanine structural analogue, having absorption and emission well into the near-IR region, has been utilized to verify energy transfer from singlet excited porphyrin to a near-IRemittingfluorophore.To achieveaxial coordination, free-base porphyrin has been functionalized with an imidazole entity at the ortho, meta, or para positions of one of the meso-

* Corresponding authors. E-mail: Francis.DSouza@wichita.edu (F.D.); nikolai.tkachenko@tut.fi (N.V.T.).

† Wichita State University. ‡ Tampere University of Technology.

10.1021/jp908115e 2010 American Chemical Society Published on Web 1/23/2009

aryl groups (see Chart 1). The different substitutions are expected to result in dyads of different orientations. Photochemical studies using both steady-state and time-resolved transient absorption techniques have been performed to probe efficiency and kinetics of excitation energy transfer in the newly formed dyads.

Results and Discussion

Optical Absorption and Binding Constant Studies. Figure

1 shows the optical absorption spectra of H2PoIm, ZnPc, and ZnNc derivatives in o-dichlorobenzene (DCB), normalized to their most intense absorption bands. The absorption spectrum of H2PmIm and H2PpIm were found to be similar to that of

H2PoIm with an intense Soret at 424 and four visible bands at 518, 553, 594, and 652 nm. The spectrum of ZnPc exhibited bands at 348, 614, 653, and 681 nm while the spectrumof ZnNc revealed peaks at 335, 690, 737, and 7 nm, respectively; that is, the spectrum of ZnNc is stretched well into the near-IR region. Importantly, the H2PIm band at 518 nm had no overlap with the absorption bands of either ZnPc or ZnNc, providing the possibility for selective excitation of the donor, free-base porphyrin.

Figure 2a and 2b show absorption spectral changes recorded during increasing addition of H2PpIm to the solutions of ZnPc and ZnNc, respectively.Similar spectral changes were observed for H2PoIm and H2PmIm binding to the acceptor zinc macrocycles (see Supporting Information Figures S1 and S2). The binding of H2PpIm to ZnPc was characterized by diminished intensity of 614 and 681 nm bands with isosbestic points at

609, 663, and 672 nm, indicating existence of only one equilibriumprocess in solution.Similarlythe binding of H2PpIm to ZnNc was characterized by diminished intensity of 690, 737, and 7 nm bands with 2-4 nm blue shifts (Figure 2b). Isosbestic points were also observed at 665, 686, 731, and 750 nm, indicating existence of only one equilibrium process in solution. Plots of method of continuous variation confirmed 1:1 complex formationbetween the donor and acceptorentities.The formation constants, K, for H2PIm binding to ZnPc and ZnNc were obtained from the absorption spectral data using the

Benesi-Hildebrand method23 (Figure 2a and 2b insets) and are listed in Table 1. The magnitude of the K values suggests stable complex formation. The K values follow the trend: para ∼ meta > ortho of imidazolesubstitutionon the phenyl ring of porphyrin macrocycle for a given zinc macrocycle binding, a trend that could be easily attributed to the steric constraints of the orthosubstitutedporphyrinderivative.Additionally,binding constants for ZnNc were found to be 2-3 times higher than that obtained for the corresponding ZnPc binding. This could be attributed to the electron-rich ZnNc macrocycle compared to ZnPc macrocycle as revealed by their electrochemical oxidation potentials, discussed in the next section.

Electrochemical Studies. Differential pulse voltammetric studies (DPV) were performed to evaluate the oxidation and reduction potential of the investigated donor-acceptor entities. The first reversible oxidation and first two reversible reductions

of H2PoIm were located at 0.5 V, and -1.67 and -2.02 V vs

Fc/Fc+ in 0.1 (TBA)ClO4, respectively.For H2PmIm and H2PpIm, the first reductionwas shiftedin the negativedirectionby 50-60 mV while the first oxidation was anodically shifted by 40 mV (see Figure 3). The first oxidation of ZnPc and ZnNc were overlappingtwo one-electronprocesses.The peak potentialwere located at -0.05 V vs -0.38 V vs Fc/Fc+, respectively, for ZnPc and ZnNc. That is, these compounds revealed easier oxidations compared to the free-base porphyrins used in the

CHART 1: Structure of the Donors and Acceptors Employed in the Present Study To Probe Excitation Energy Transfer

Figure 1. Absorption spectra of (i) H2PoIm, (i) ZnPc, and (ii) ZnNc in DCB, normalized to their most intense bands. The concentrations are in the range of 5-10 µM.

Ultrafast Singlet-Singlet Energy Transfer J. Phys. Chem. A, Vol. 114, No. 1, 2010 269

present study. The first reduction peaks of ZnPc and ZnNc were located at -1.65 and -1.70 V vs Fc/Fc+, indicating these to be poor electron acceptors (vide infra).

Energy Optimization by DFT Calculations. Since the relative orientation of the donor and acceptor dipoles in the dyads is crucial for energy transfer efficiency, the structures of the supramolecular dyads were visualized by performing computational studies at the B3LYP/3-21G(*) level.24,25 Figure 4 shows the structures of the dyad optimized on a Born- Oppenheimer potential energy surface. The two macrocyclic rings of the H2PoIm:ZnPc and H2PoIm:ZnNc dyads were found to be in a skipped coplanar arrangement while for the meta and para imidazole derivatized dyads, H2PIm:ZnPc and H2PIm: ZnNc, they were positioned at an angle less than 90°. The edge- to-edgedistances,center-to-centerdistances,and angles between the two macrocycle planes of the donor-acceptor entities are listed in Table 1. Generally, the distances varied as follows: ortho < meta < para imidazole-substituted porphyrins. That is, a closerdistanceand skippedcoplanarconfigurationfor the ortho derivatives,and a relatively longer distance and near-orthogonal configurationfor the meta and para substitutedderivatives,were observed.

The frontierorbitals,HOMO and LUMO,were also evaluated for both types of dyads, and the representative orbitals for selected dyads are shown in Figure 4. It is important to note that the HOMO for all of the studied dyads was fully localized on the ZnPc or ZnNc macrocycleswhile the LUMO was located on the porphyrin macrocycle. In conjunction with the earlier discussed electrochemical results, these results point out that electron transfer from the excited free-base porphyrin to either the coordinated ZnPc or ZnNc is less likely to take place. The gas-phase HOMO-LUMO gap is found to be smaller by ∼300 mV for the H2PIm:ZnNc series of dyads compared to the

H2PImZnPc series of dyads, a result that readily agrees with the electrochemical results.

Figure 2. UV-visible-near IR spectral changes observed during increasing addition of H2PpIm (0.1 equiv) to a solution of (a) ZnPc (0.1 mM) and (b) ZnNc (0.1 mM) in DCB. The figure insets show Benesi-Hildebrand plots constructed to obtain the binding constants. The absorption changes of the 681 nm band of ZnPc and 776 nm band of ZnNc were utilized.

TABLE 1: Binding Constants, K, and B3LYP/3-21G(*) Computed Results for Dyads Formed by Binding Imidazole-Appended Free-Base Porphyrin to Zinc Phthalocyanine or Zinc Naphthalocyanine dyada K, mol-1 b center-to-center distance, Å edge-to-edge distance, Å angle between rings, deg HOMO-LUMO gap, eV a See Chart 1 for the structure of different donor and acceptor entities; in DCB at room temperature. b Error )( 10%.

Figure 3. Differential pulse voltammograms of the investigated compounds (∼0.5 mM) in DCB containing 0.1 M (TBAP)ClO4. DPV conditions: scan rate ) 20 mV/s, pulse width ) 50 ms, step time )

100 ms, and pulse height ) 0.025 V.

270 J. Phys. Chem. A, Vol. 114, No. 1, 2010 Maligaspe et al.

Using the electrochemical,computational,and excited energy data, the free energies of charge-separation (∆GCS) were calculated using eq 1 by Weller’s approach.26 where ∆E0-0 is the energy of the lowest excited state of H2P

(1.89 eV), ∆GS )- e2/(4πε0εRRCt-Ct), and ε0 and εR refer to vacuum permittivity and dielectric constant of DCB.

The calculations revealed ∆GCS values to be endothermic by 0.1-0.2 eV for electron transfer from the singlet excited free- base porphyrinto eitherZnPc or ZnNc, suggestingthe less likely occurrence of such reactions in the studied dyads.

Steady-State Fluorescence Studies: Singlet-Singlet Energy Transfer. As pointed out earlier, excitation of the donor,

H2PIm, at 518 nm to a large extent selectively excites the freebase porphyrin, thus allowing us to monitor energy transfer to the acceptor,ZnPc or ZnNc entities,without them being directly getting excited. Figure 5a and 5b show the spectral changes observed for H2PpIm emission during increasing addition of

ZnPc and ZnNc in DCB. The emission band of H2PpIm located at 655 nm revealed quenching with simultaneous appearance of new emission bands at 694 and 758 nm corresponding to ZnPc (Figure 5a), and 782 and 820 nm corresponding to ZnNc

(Figure 5b). Similar results were obtained when H2PmIm and H2PoIm were titrated with either ZnPc or ZnNc (see Supporting

Information for the spectral data, Figures S3-S6). Further, the excitation spectra of the dyads were recorded by holding the excitation wavelength at 758 for ZnPc and 782 for ZnNc. Such spectra revealed absorption bands corresponding to both donor and acceptorentities(see SupportingInformationfor the spectral data, Figures S7). In a control experiment, free-base meso- tetraphenylporphyrin,H2TPP, was also titratedwith the acceptor molecules. Under these conditions, only a slight increase of the acceptor emission in the near-IR region was observed. These results confirm occurrence of singlet-singlet energy transfer in the self-assembled dyads.

Figure 5c and 5d shows the extent of energy transfer for each of the porphyrin derivatives upon increasing addition of ZnPc and ZnNc, respectively. It is clear from these plots that after addition of about 3-4 equiv of the acceptor entities, the energy transfer has attained its maximum value. Further, the extent of energy transfer followed the order: H2PpIm > H2PmIm > H2PoIm . H2TPP according to their binding constants. The latter plots for H2TPP interactions being virtually horizontal imply occurrence of little or no energy transfer.

ExcitedEnergyTransfer:TheoreticalConsiderations.The observed excitation energy transfer (EET) could occur either via Dexter’s exchange mechanism or Forster’s dipole-dipole mechanism. The former mechanism requires the presence of electronic communication between the donor and acceptor species (via orbital overlap).27 However, the frontier orbitals from the DFT studies (Figure 4) in conjunction with the

Figure 4. B3LYP/3-21G(*) optimized structures of the dyads formed via axial coordination of H2PIm (ortho, meta, or para) to (a) ZnPc and (b) ZnNc. The HOMO and LUMO of H2PmIm bound to ZnPc and ZnNc are shown in the lower panels.

Ultrafast Singlet-Singlet Energy Transfer J. Phys. Chem. A, Vol. 114, No. 1, 2010 271

spectroscopicstudiesreveal that such intramolecularinteractions are almostnonexistent.Therefore,the resultsof the presentstudy have been analyzed according to Forster’s mechanism. According to this mechanism, the rate of excitation energy transfer, kForster, is given by eq 2.

where n is the solvent refractive index, ΦD and τD are the fluorescence quantum yield () 0.12) and the fluorescence lifetime of the isolated donor (free-base porphyrin), JForster is the Forster overlap integral representing the emission of the donor and absorption of the acceptor ZnPc or ZnNc, and R is the donor-acceptor center-to-center distance (Table 1). The τ values measured using a strobe technique were found to be 1.50, 9.95, and 9.45 ns, respectively, for the ortho, meta, and para imidazole-derivatized free-base porphyrins, values close to those reported for free-base tetraphenylporphyrins in the literature.1 In eq 2, κ2 is the orientation factor as described in eq 3, often playing a key role in determining the directionality of excitation energy transfer.

where R and are the angles made by the transition dipoles of the donor and acceptor entities with the line joining the centers of the transitions, and ν is the angle between the two transition dipoles. The transition dipoles of tetrapyrroles are known to lie along a line joiningtwo opposingpyrrolenitrogens.28 Depending upon the relativeorientationof the donor and acceptor,the value of κ2 could range from 0 to 4. For head-to-tail parallel transition dipoles, κ2 ) 4, for parallel dipoles, κ2 ) 1, and for dipoles oriented perpendicular to each other, κ2 ) 0. An analysis of the results presented in Figure 4 shows different orientations for the ortho, meta, and para substituted derivatives. However, considering the flexible nature of the connecting axial bond, a value of κ2 ) 2/3, generally used for randomly oriented dipoles, is employed.

(Parte 1 de 3)

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