Compact Self-Assembled Porphyrin Macrocycle - American Chemical

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Compact Self-Assembled Porphyrin Macrocycle: Synthesis, Cooperative Enhancement, and Ultrafast Response Oleg Varnavski,† Jeffery E. Raymond,† Zin Seok Yoon,⊥ Takefumi Yotsutuji,‡ Kazuya Ogawa,*,‡,§ Yoshiaki Kobuke,*,‡,∥ and Theodore Goodson, III*,† †

Department of Chemistry and Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States ⊥ Samsung Display, Samsung st 181, Tangjeong-Myeon, Asan-City, Chungcheongnam-Do 336-741, South Korea ‡ Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan § Interdisciplinary Graduate School of Medicine and Engineering, Division of Medicine and Engineering Science, Life Environment Medical Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan ∥ Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: In this study, we present the synthesis of a carbazole-bridged porphyrin dimer system that possesses a 90° change in orientation between porphyrin units and the single-product four dimer macrocycle, which is formed upon self-assembly of the dimer via imidazolyl-to-zinc complementary coordination. Subsequent characterization of the two-photon absorption and ultrafast emission lifetimes of these systems indicates a very strong coupling between constituent dimers in the assembled macrocycle structure. These interactions lead to a red-shifted two-photon response and a full order of magnitude increase in the two-photon absorption (TPA) cross section per building block with respect to the lone dimer. Excitonic coupling through the slipped cofacial arrangement created by imidazolyl-to-zinc interaction has been shown to play a critical role in the observed TPA enhancement. This points to the use of our small lightharvesting mimic for nonlinear optical applications in which aggregation effects have often stymied development.

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INTRODUCTION Organic nanomaterials for advanced optoelectronic applications with high tunability, extraordinary response, or novel chemistries are becoming increasingly important.1−4 Multichromophore and self-assembly approaches have been applied to the creation of superior light-harvesting, light-emitting, and nonlinear optical materials.3−5 It has been shown that advanced architectures such as dendrimers, molecular wires, and large macrocycles can offer significant advantages over other structures for multiphoton and energy-harvesting applications.2,6−11 Bioinspired model systems constitute a substantial part of the efforts directed to the realization of highly efficient molecular photonic and electronic devices such as molecular wires, switches, transistors, and artificial light conversion apparatus to exploit the high efficiency of natural systems.12−15 Since the discovery of the structure of efficient light-harvesting devices such as that in purple bacteria (LH1 and LH2), many research groups have devoted effosrt to mimicking them.9,10,13 The most common and versatile chromophore used in such studies is the porphyrin.9,10,12,16 Among these chromophores, cofacial porphyrin compounds exhibit unique properties because of their spatial arrangement with respect to each other and the ability to arrange multiple porphyrins into ordered supramolecular arrays.10,12,16 This particular arrangement provides a way to place chromophores at a given distance providing a short pathway for interchromophore interactions © 2014 American Chemical Society

and communications. Naturally occurring complexes such as LH1 and LH2 have been constructed using coordination of imidazolyl to Mg centers in chlorophylls with no use of covalent bonding.17 They exhibit ring-like-shaped protein structures in which almost cofacial arrangements (similar to slipped dimers) of the chlorophyll chromophores occur.17 While various covalently bonded porphyrin arrays (as well as assemblies) built upon coordination linkages were investigated, the interchromophore interactions and excited state structure of self-assembled porphyrin arrays utilizing the specific coordination of imidazolyl to the metal (and closely mimicking the natural coupling motif) are not well understood. Strong cooperative enhancement of the two-photon absorption cross section per porphyrin unit has been observed in large macrocycles based on the same imidazonyl−porphyrin selfcoordination motif.10 The observed TPA enhancement indicates a strong interchromophore coupling extending over the distances similar to the size of LH complexes.10 However, for processes in which the solubility and size of the assembly are limiting factors to device design, obtaining TPA cross section (δ2) enhancement with the minimal architectural size is greatly important.18 To address these concerns, we offer here Received: September 17, 2014 Revised: November 3, 2014 Published: November 7, 2014 28474

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Scheme 1. Synthesis of a Carbazole-Bridged Zn Porphyrin Dimer and Subsequent Self-Assemblya

a

Porphyrin cycles are separated out of plane at pyrrole ring in schemes 6R and 7R. The space-filling models are not optimized.

the synthesis and two-photon and ultrafast emission characterization of a porphyrin dimer that provides a 40-fold δ2 enhancement upon self-assembly to a square macrocycle; this assembly can then be covalently bound to provide a robust architecture that is viable in multiple environments. Unlike species in which self-assembly generates a broad range of sizes,10 only a single macrocycle species can be obtained with the carefully selected 90° carbazole bridge (Scheme 1).

The cyclic four-unit system (6R) was allowed to selfassemble over time.10,19,20 This was accomplished with a solution of 113 μM 6 in 30 mL of a chloroform/methanol [7:3 (v/v)] mixture that was subsequently allowed to age for 24 h at room tempurature without being exposed to light. The solution was evaporated and subjected to preparative GPC with chloroform elution to give pure 6R (58%). The analytical GPC of 6R kept in chloroform as shown in Figure S2 of the Supporting Information showed only tetramer after several days. This indicates that the tetrameric sctrucute of 6R is very stable in chloroform for a long time without producing any other species such as a pentamer, hexamer, or larger polymer. To determine the exact molecular weights of the cyclic tetramer by mass measurement, we fixed the coordination structure via a ring-closing metathesis reaction of meso substituents to

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EXPERIMENTAL METHODS The porphyrin dimer (6) was synthesized from ethynylimidazolylporphyrin (5) and 3,6-diiodo-9-(3,5,5-trimethyl)hexylcarbazole (2) using a Pd2(dba)3/AsPh3 catalytic system (Supporting Information). The resultant dimer was isolated using preparative GPC with pyridine elution in 30% yield. 28475

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configuration (151 cm−1), which can be related to the weaker interchromophore interaction in the relaxed configuration. This may result in more localized fluorescence states observed in other macromolecular systems.7,13,21 A larger bathochromic shift for the absorption spectrum is related to a smaller Stokes shift (reorganization energy) for the macrocycle (159 cm−1) in comparison with that of the dimer (209 cm−1) . This effect can be associated with the suppression of the torsional motion in the macrocycle due to the cofacial arrangement in the macrocycle. Similar effects (suppression of torsional motion and decreasing Stokes shift in conjugated porphyrin oligomers when a metal-coordinating template restricts the torsional motion) have been reported by Hertz at al.9 A smaller reorganization energy for the delocalized state of the macrocycle (motional narrowing)22 can also contribute to a smaller Stokes shift for the macrocycle compared to that of the dimer. B band splitting (observed as a blue shoulder) for the dimer (6) is smaller than that reported for the m-bis(ethynylene)phenylene-linked zinc-imidazolylporhyrin Zn-EP-Zn.12 Weaker B band coupling for the carbazole-bridged dimer (6) can be associated with the fact that the primary dipole moments of the covalently bound porphyrins are orthogonal to one another, while in the case of Zn-EP-Zn, the angle between the dipoles is 120°, providing stronger dipole−dipole interaction. The two-photon response of assembly 6R displayed a surprisingly strong cooperative enhancement of the TPA cross section (a factor of ∼40) with respect to that for the covalently bound dimer (Figure 2). A summary of the TPA properties of

produce 7R. After the reaction, GPC separation was performed again. Matrix-assisted laser desorption ionization time-of-flight spectra of the main fraction showed a sharp ion peak at 7096.48 corresponding to the molecular weight of the four-dimer macrocycle. Time-resolved and nonlinear spectroscopic measurements were performed on the 6R macrocycle. Femtosecond time-resolved fluorescence measurements were taken using a fluorescence upconversion system with excitation provided by a frequency-doubled light from a mode-locked Tisapphire laser (Tsunami, Spectra-Physics) at 800 nm. Fluorescence emitted from the sample was upconverted in a nonlinear crystal of β-barium borate using a pump beam at 800 nm, which first passed through a variable delay line. The instrument response function [IRF (close to Gaussian with σ = 102 fs)] was measured using Raman scattering from water. Spectral resolution was achieved by using a monochromator and photomultiplier tube. More details of the upconversion setup are provided elsewhere7,22 (see the Supporting Information). Two-photon absorption cross sections have been measured through the two-photon excited fluorescence method (TPEF) that has been outlined elsewhere in detail.23−26 Excitation was provided by a Mai-Tai (Spectra-Physics) laser source, which could be tuned from 690 to 1020 nm and was operated at 80 MHz with 110 fs pulses. Data collection was performed using an Oriel Cornerstone (Newport) monochromoter and a Hamamatsu C9744 photocounting unit. Rhodamine B in methanol (10 uM) was used as the standard.

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RESULTS AND DISCUSSION Steady state absorption spectra shown in Figure 1 indicate a red shift in the Q line absorption peak for the macrocycle species

Figure 2. Two-photon cross section of 6 and 6R per dimer. The dashed line is a guide for the eye.

Figure 1. Normalized absorption and fluorescence spectra of the dimer (6) in pyridine and the macrocycle (6R) in chloroform.

the dimer 6 (in pyridine) and macrocycle 6R (in chloroform) can be found in Table 1, while two-photon absorption spectra are presented in Figure 2. These results were obtained through the two-photon excited fluorescence method (TPEF).23−26 While the dimer (6) showed a moderate TPA response that is comparable to that observed from other π-bridged Zn− porphyrin dimers,10,27 the ensemble of four dimers as a macrocycle demonstrated an impressive TPA cross section rise. Cooperative superlinear enhancement of the TPA response (10-fold rise per dimer) is undoubtedly the result of strong interdimer interaction within the macrocycle. This is in qualitative agreement with the bathochromic spectral shifts observed in steady state linear spectra.

compared to that of the dimer. The observed systematic red shifts are consistent with B and Q states being delocalized beyond the size of the dimer because of strong interdimer coupling. This coupling can be attributed to a tight slipped cofacial arrangement provided by the pendant imidazolyl at the terminus of each porphyrin (Scheme 1).20 The bathochromic shift is stronger for the B band (485 cm−1) than for the Q band (210 cm−1), indicating stronger interdimer coupling for the B band than for the Q band. Interestingly, the bathochromic shift for the Q band in absorption (Franck−Condon) configuration (210 cm−1) is stronger than that for the fluorescence (relaxed) nucleus 28476

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Table 1. Summary of Steady State, Two-Photon, and Fluorescence Dynamics Measurements S1 (Q band) dynamics species dimer (6) macrocycle (6R) a

absorbance maximum (nm)

PL maximum (nm)

450, 651 460, 660

660, 717(s) 667, 723(s)

ΦPL

δ2 (GM)a

δ2/dimer (GM)a

τ1 (ps)

τ2 (ps)

anisotropy decay τR (fs)

residual anisotropy rres

0.15 0.14

720 28680

720 7170

0.07 (rise time) 0.64 (decay time)

213 6.89

80 30

0.075 0.06

One GM = 10−50 cm4 s/photon.

where n is the refractive index, h Planck’s constant, c the speed of light, L the local field factor [L = (n2 + 2)/3 for a simple uniform isotropic model], φ the angle between transition moments μgi and μif, and Γf the line width of the f state (TPA transition). It is seen in eq 1 that δ2 can be expressed as a product of several terms depending on different environmental and material parameters. From TPA and linear absorption spectra, the mismatch factor for the macrocycle can be estimated to be 52. At the same time, the mismatch factor for the dimer is near 54, very similar to that for the macrocycle. This means that the change in the mismatch factor when going from the dimer to the macrocycle is not responsible for the ∼40-fold enhancement of TPA for the macrocycle. The transition dipole factor [(|μgi|2|μif|2)/Γf] in eq 1 is closely related to the wave function delocalization length L, and we name it the “delocalization factor”. For example, for a fully conjugated system with conjugation length Lc, this factor is expected to be proportional to Lc4 because of the transition moment scaling relation μ ∝ LC. However, for the macrocyclic system described in this work, the conjugation is not expected to expand beyond the dimer due to specifics of imidazolyl−Zn coordination coupling (the HOMO orbital density at the Zn center is expected to be near zero). Therefore, the excitonic (through space) coupling should be considered for this system. For Frenkel exciton systems (where the electron−hole pair is localized on one chromophore), the transition moment scales as μ ∝ √LF, where LF is a delocalization length of the Frenkel exciton.32 For the case of a fully delocalized exciton, it translates to the scaling factor μ ∝ √N, where N is the number of strongly coupled monomers. Therefore, the delocalization factor for the TPA cross section is proportional to N2 if the excitation is delocalized over the entire macromolecule, which is in accordance with general χ(3) size scaling factors in molecular

To better understand the origin of the observed TPA enhancement, we will use a three-level model for the TPA cross section that was successfully applied to other porphyrin-based systems.28,29 The energy level diagram related to the three-level model system used for the description of the TPA response is depicted in Figure 3.

Figure 3. Energy level diagram for the porphyrin arrays. g is the ground state. i is the intermediate Q state. f is the two-photon allowed final state.

Using the sum-overstates expression derived from a perturbation theory30 for a molecule with three essential states and with averaging over molecular orientations,31 the peak TPA cross section δ2 can be given by29

Figure 4. Fluorescence decay profiles. (a) Q band (S1) emission for the dimer and macrocycle at 683 nm. (b) Short time scale fluorescence dynamics for the dimer (6). 28477

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aggregates.33 In this case, the TPA cross section per chromophore can be assessed as follows: (δ2/N) ∼ N if other factors (orientational, local environment, and mismatch factors) do not change significantly when going from the monomer to large macrocycles. If the excitonic coupling is strong enough in our case to maintain excitonic states delocalized over the macrocycle, the factor |μgi|2|μif|2 can lead to 16-fold enhancement, which is still smaller than that found in the experiment [∼40 (Table 1)]. From our experiments, the line width Γf of the TPA cross section (not very accurate because of the relatively large wavelength increment) can be estimated to be approximately the same for the dimer and the macrocycle. Therefore, it does not substantially contribute to the observed enhancement factor. It is also worth noting that the TPA line width was found to be weakly dependent on the porphyrin dimer structure for covalently coupled dimers and can hardly produce an additional enhancement of a factor of >2.29 Different transition dipole moment mutual orientations of dipoles μgi and μif for the dimer and macrocycle can contribute to the TPA enhancement through the orientational factor (eq 1). In this case, there is a perpendicular orientation of transition moments μgi and μif for the dimer (6) and a near parallel orientation in symmetrical macrocycle 6R; hence, the orientation factor can lead to an additional enhancement factor of 3, which may explain the experimentally observed trend in the TPA cross section. Regardless of the exact model, large TPA cooperative enhancement upon assembly of the dimers into the macrocycle clearly requires a strong interdimer coupling, above the level that is responsible for a simple incoherent hopping process, as observed in some Zn−porphyrin aggregated systems.34−37 To further address the nature of the interdimer coupling and better understand how the excited state behavior may relate to δ2 enhancement, femtosecond time-resolved fluorescence upconversion measurements were performed. Isotropic (magic angle) fluorescence decay profiles for the Q band emission of the macrocycle and the dimer are shown in Figure 4. We have not found any excitation intensity dependence of the decay profile within the intensity range (tens of picojoules per pulse) used in our fluorescence upconversion setup. The fluorescence decay profile of the dimer (6) probed at 683 nm after photoexcitation (Figure 4b) exhibits a rise component with a 70 fs rise time constant corresponding to a relatively fast S2−S1 internal conversion process (this conversion time can be compared with a conversion time of ∼1.4 ps for Zn− tetraphenylporphyrin35,39). No detectable rise time component has been deduced from the fluorescence profiles of macrocycle 6R, indicating the S2−S1 internal conversion process proceeds much faster than that for the dimer. This increased internal conversion rate can be related to the presence of an excitonic band in the macrocycle assembly between the dimeric B band and Q band that can promote an additional ladder-type relaxation channel due to an increased overlap integral between the electronic B states and high-lying vibronic levels of the Q state.35,36 Longer time scale dynamics of the fluorescence depicted in Figure 4a shows the presence of a shorter picosecond component in the fluorescence of macrocycles as compared to that for the dimer building block (Table 1). A similar trend has been previously observed for different aggregated systems based on Zn−porphyrin.36,38 The exact origin of these additional fast components in the S1 decay is still a matter of discussion.36,38−41 Several mechanisms have been suggested,

including conformational changes35,38,40,41 and vibrational cooling.36,39 It is worth noting that in our case the transient absorption experiment with the excitation (pump) to the Q line (λexc = 655 nm) showed the presence of a similar ∼10 ps component in the ground state bleach decay (Figure S6 of the Supporting Information). This result shows that the internal S2−S1 conversion process is not responsible for the appearance of the picocecond component in the S1 decay profile of the assembled system 6R. The vibrational cooling contribution should also be minor as the pump in this case is just slightly above the band edge, resulting in a rather small vibtrational energy being delivered to the system. The fluorescence quantum yield for the macrocycle has been found to be essentially the same as that for the dimer [0.14 vs 0.15 (Table 1)], which is similar to the trend found for the mbis(ethynylene)phenylene-linked zinc−imidazolylporphyrin systems (dimer vs pentagonal and hexagonal arrays) utilizing the same slipped cofacial interaction mechanism.12 The similarity in fluorescence quantum yields makes the presence of new fast nonradiative deactivation channels related to the observed picosecond decay components in macrocycles less probable. Conformational dynamics, specific to the assembled system, may be responsible for the presence of fast picosecond components in the assembled system. However, the transient absorption experiment did not show any ground state bleach spectral dynamic shift characteristic of a degree of torsional movements40,41 within our signal-to-noise range (Figure S6 of the Supporting Information). This is in line with a suggestion of the suppression of the torsional motion for the macrocycle discussed above. Further investigation is needed to better understand the mechanism behind the picosecond decay component in the S1 fluorescence in the assembled system. It is known that time-resolved fluorescence anisotropy experiments can provide valuable information about the excitation energy migration and interchromophore coupling in multichromophoric molecular systems.6,26,42−46 Experimental fluorescence anisotropy R(t) was calculated from the decay curves for the intensities of fluorescence polarized parallel [Ipar(t)] and perpendicular [Iper(t)] to the polarization of the excitation light according to the equation R(t) = (Ipar − GIper)/ (Ipar + 2GIper). The factor G accounts for the difference in sensitivities for the detection of emission in the perpendicular and parallel polarized configurations. Figure 5 shows the fluorescence anisotropy decay for macrocycle 6R. Fast initial anisotropy decay around the instrument response profile, followed by a small residual, relatively long-lived anisotropy, can be seen. The residual anisotropy value for fluorescence (∼0.06), prior to rotational diffusion, is lower than that expected for the planar equilibrated system (∼0.1).42,45 This can be a signature of an incline of the transition moment of the fluorescent state from the common molecular plane13,45 or a distortion of the molecule’s shape in the relaxed fluorescent excited state from a perfect planar configuration (see Scheme 1). The longest decay component was >100 ps, and it contributed to a nearly flat residual value on a short time scale. Considering the size of the macrocycle and the viscosity of the chloroform (∼0.54 cP at 25 °C), this component can be associated with the rotational diffusion of the system. We have performed a best fit of the fluorescence anisotropy data with multiexponential decays after assuming the convolution of the IRF with the parallel and perpendicular absorption intensity profiles (see the Supporting Information). 28478

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version of the Redfield approach for multichromophoric ring systems.47 In this model, the discussion of the coherence of excitations is led by using an expression in which the limiting cases of Förster transfer (weak interaction) and completely delocalized excitonic states (strong interaction) are continuously connected to one another.47 This is a simplified model for the conjugated macrocycles used in this work, but it allows for a crude initial estimation of the intracycle coupling strength (transfer integral) and the excitation energy migration regime. Modeling anisotropy decay time as a function of the interaction strength via this approach showed a strong coupling (Frenkel exciton) regime with an interaction strength of ∼1000 cm−1 (Figure S7 of the Supporting Information), which leads to the excitation localized over the entire macrocycle. This result correlates well with the strong cooperative enhancement of the TPA absorption described above. Time-resolved fluorescence anisotropy for the dimer is shown in Figure 7. Because of the 90° arrangement of the arms

Figure 5. Q band fluorescence anisotropy dynamics for macrocycle 6R. Parallel and perpendicular polarized fluorescence profiles are shown in the inset. Best fit results are shown as solid lines. The instrument response function (IRF) is also shown as a dashed−dotted line.

The fluorescence anisotropy of the macrocycle on the time scale of a few picoseconds was best modeled with the fast exponential decay followed by a long-lived residual component. The fast decay component in each macrocycle was found to be 30 ± 20 fs. A relatively large uncertainty in the decay time is associated with the substantial noise in the vicinity of the IRF. We should also note that in spite of this noise figure any attempt to model the experimental anisotropy decay with a exponential decay time of >50 fs was not successful. Different experimental noise realizations were tested. The ultrafast depolarization is associated with strong interchromophore interaction and formation of a delocalized state over a substantial portion of the macrocycle.13,26,46 It is important that no signs of the picosecond decay component have been observed in fluorescence anisotropy on a longer time scale (but before rotational diffusion) as shown in Figure 6.

Figure 7. Time-resolved fluorescence anisotropy of the dimer (6) in pyridine. The detection wavelength is 683 nm. The best fit is shown as a solid line. The instrument response function (IRF) is also shown as a dashed−dotted line.

of the dimer, the residual anisotropy before rotational diffusion depends on the relative contributions of the excited states polarized in perpendicular directions and can potentially be as low as −0.2 at particular excitation wavelengths. With the excitation of 400 nm used in our experiment, the residual anisotropy before rotational diffusion was found to be 0.075. The best fit procedure with a single-exponential anisotropy decay function results in an anisotropy decay time of 80 ± 10 fs. This ultrafast anisotropy decay time is reasonable taking into account coupling of the Zn−porphyrins via the conjugated carbazole bridge. It is worth noting that in spite of the relatively high noise the best fits in different experimental scans (noise realizations) always have resulted in systematically slower anisotropy decay in comparison with those for the macrocycle. The best fit procedure has always come up with slightly higher residual anisotropy values. These observations indicate an additional interaction mechanism in the macrocycle leading to the faster anisotropy loss and a lower residual value. These results can be discussed in terms of the two types of interporphyrin coupling that occur in macrocycle 6R. The first is the conjugated bridge between the porphyrin arms of the dimer. Unlike J aggregation in linear assemblies, the primary dipole moments of the covalently bound porphyrins are orthogonal to one another in the dimer and directed toward the bridging carbazole (Figure 1). The second coupling motif is

Figure 6. Time-resolved fluorescence anisotropy decay profile of 6R on a picosecond time scale.

Such picosecond anisotropy dynamics are often indicative of a slow hopping process in multichromophore arrays.7,13,37 The lack of a slow hopping component supports the concept of excitation delocalized over the entire macrocycle with no room for hopping left. Simple qualitative analysis of the interchromophore interaction regime and the electronic coupling within the macrocycles can be provided by a phenomenological 28479

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The Journal of Physical Chemistry C related to the slipped cofacial arrangement of Zn−porphyrins due to imidizolyl−zinc interaction. Our investigation showed that the last type of coupling motif is the most critical for the cooperative effect in TPA enhancement in spite of the fact that it has an excitonic origin not related to the conjugation. Other Zn−porphyrin systems based on this coupling motif also showed very large TPA cross sections in the range of thousands to tens of thousands of GM.10,18,28,48,49 The combination of both types of interactions in a single small macrocycle, supported by steady state, time-resolved, and TPA spectroscopy, strongly suggests an environment that allows for enhanced nonlinear optical responses. The compact arrangement of the “square” macrocycle may lead to an advantageous balance of the “closest neighbor−neighbor” and “across the cycle” interactions leading to the overall increased level of interchromophore coupling in the ring. Strong interchromophore coupling resulting in the excitonic state delocalized over the major part of the compact porphyrin “square” supported by our results may lead to the creation of morphologically (dense packing) and physically (strong dipolar interaction) wellmatched building blocks for efficient energy light harvesting and nonlinear optical applications.

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REFERENCES

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CONCLUSION In this study, we present the total synthesis of a zinc−porphyrin dimer capable of selective self-assembly into four-unit macrocycles. For the lone dimer in solution, we report a δ2 of 720 GM/dimer. Upon assembly of the macrocycle species, a full order of magnitude two-photon absorption enhancement is observed providing a δ2 of 7120 GM/dimer. Ultrafast lifetime and anisotropy studies confirm that this enhancement corresponds to a significantly greater degree of transition dipole interaction between porphyrins dependent largely on the self-assembled architecture. This study clearly shows that a slipped cofacial arrangement via imidazole−zinc interaction, while having noncovalent (excitonic) character, provides exceptionally strong two-photon response enhancement resulting in TPA cross sections in the range of ∼3 × 104 GM for the macromolecular system of modest size (eight Zn− porphyrin units). This result is comparable with the best TPA response for covalently linked conjugated porphyrin arrays29 while offering significant advantages and flexibility associated with the self-assembly manufacturing process. This synthesis and spectroscopic study presents an architecturally driven method by which the rapid generation of organic two-photon enhanced materials can be achieved, a property critical to the applications of next-generation nonlinear and quantum optical materials. ASSOCIATED CONTENT

* Supporting Information S

Detailed synthesis, experimental procedures, and associated spectrometric data. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Photochemistry, via Grant DE-SC0012482.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 28480

dx.doi.org/10.1021/jp509439g | J. Phys. Chem. C 2014, 118, 28474−28481

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