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Photophysics of Self-Assembled Zinc Porphyrin-Bidentate Diamine Ligand Complexes Brook R. Danger, Krysta Bedient, Manisankar Maiti, Ian J. Burgess,* and Ronald P. Steer* Department of Chemistry, UniVersity of Saskatchewan, 110 Science Place Saskatoon SK, Canada S7N 5C9 ReceiVed: July 21, 2010; ReVised Manuscript ReceiVed: September 1, 2010
The effects of complexationsby bidentate nitrogen-containing ligands such as pyrazine and 4,4′-bipyridine commonly used for porphyrin self-assemblyson the photophysics of the model metalloporphyrin, ZnTPP, are reported. Ligation to form the 5-coordinate species introduces an intramolecular charge transfer (ITC) state that, depending on the oxidation and reduction potentials of the electron donor and acceptor, can become involved in the excited state relaxation processes. For ZnTPP, ligation with pyridine has little effect on excited state relaxation following either Q-band or Soret band excitation. However, coordination of ZnTPP with pyrazine and bipyridine causes the S2 (Soret) state of the ligated species to decay almost exclusively via an S2-ICT-S1 pathway, while affecting the S1 decay route only slightly. In these 5-coordinate species the S2-ICT-S1 decay route is ultrafast and nearly quantitative. Literature redox data for other bidentate ligands such as DABCO and multidentate ligands commonly used for pophyrin assembly suggest that the ITC states introduced by them could also modify the excited state relaxation dynamics of a wide variety of multiporphyrin arrays. Introduction Porphyrins, metalloporphyrins, and related macrocycles are widely employed as the chromophoric species in a diverse array of photon-actuated supramolecular structures, including lightharvesting dendrimers,1 molecular tweezers,2 double-strand ladder-type molecular wires,3 several types of sensors,4 and other photonic devices.5 In many of these applications, the metalloporphyrin moieties are self-assembled by interaction with bidentate or multidentate nitrogen-containing ligands that coordinate noncovalently with the metal(s) in the axial position(s). For those applications involving photon actuation, it is therefore important to establish the photophysical effects of complexation by these types of ligands on the porphyrin chromophore in its several excited electronic states. Metalloporphyrins have also been used as photon-harvesting moieties in dye-sensitized solar cells (DSSCs),6 and interest in this application has been rekindled as a result of recent research on noncoherent photon upconversion (NPU).7 In most organic systems investigated to date, photon upconversion involves absorption at λ1 by a compound that produces long-lived triplet states in high yield, followed by triplet-triplet energy transfer to a second component that then undergoes triplet-triplet annihilation (TTA) and fluoresces from its S1 product state at λ2 < λ1. Appropriate choices of absorber and emitter can produce excited singlet states that yield upconverted fluorescence with reasonable efficiency under modest illumination intensities.8 This observation has led to proposals for incorporating NPU into various photonic devices,9 sensors,4 and DSSCs.7 Although the mechanistic details are not yet fully elucidated,8,10 it is clear that if chromophore self-assembly is well controlled, NPU can produce a significant quantum yield of excited electronic states at energies higher than that of the absorbed photons in both polymers and homogeneous fluid solutions under typical solar illumination intensities. The possibilities for harvesting energy in the photon-rich red and near-infrared regions of the solar * To whom correspondence should be addressed. E-mail: ian.burgess@ usask.ca (I. J. B.),
[email protected] (R. P. S.).
spectrum and then using NPU to produce excitonic states that are capable of charge transfer/separation in organic solar photovoltaics are intriguing and under active investigation.7,8,10 We have recently shown that NPU can be observed in deoxygenated fluid solution10,11 and in thin PMMA films12 containing ZnTPP or other metalloporphyrins as the only chromophoric component. Illumination at 532 nm in the Q bands produces readily measurable fluorescence from the upper S2 (Soret) state of the dye as a result of efficient 2T1 f S2 + S0 annihilation. The relative efficiency of TTA observed in these media varies with the negative exponent of the interplanar spacing between pairs of interacting triplet porphyrins, as required by the short-range (intermolecular wave function overlap) nature of the process. Prior aggregation of the metalloporphyrin is therefore required for TTA to occur in these onechromophore thin film systems. We have also shown10a that NPU can occur by heteromolecular three-center TTA in selected twocomponent systems in fluid media in which the triplet energy of the second component is higher than that of the photon absorber. The above information makes it clear that knowledge of the photophysical effects of the ligation of chromophores by molecules commonly used for their self-assembly is crucial to understanding and predicting the behavior of these aggregates and supramolecular arrays. In the present paper we describe experiments designed to measure the effects on the photophysical behavior of diamagnetic metalloporphyrins produced by complexing them with bidentate nitrogen-containing noncovalent ligands that have frequently been used previously for porphyrin self-assembly.1-3 In light of previous studies of photoinduced electron transfer in covalently linked metalloporphyrin-acceptor dyads13 and triads,5a we pay particular attention to the excited state redox properties of these noncovalent complexes. We use ZnTPP as the model metalloporphyrin, pyrazine and 4,4′bipyridine as the model ligands, and examine the photophysical characteristics of these systems when the porphyrin is excited to both its S1 (Q-band) and S2 (Soret band) states. With these data in hand, comparisons with the results of previous experi-
10.1021/jp106809j 2010 American Chemical Society Published on Web 09/24/2010
Self-Assembled Zn Porphyrin-Bidentate Diamine Complexes ments exploring the photophysical effects on metalloporphyrins of monodentate aromatic ligands such as pyridine and bidentate aliphatic diamines such as 1,4-diazabicyclo[2,2,2]octane (DABCO) become possible.
J. Phys. Chem. A, Vol. 114, No. 41, 2010 10961 TABLE 1: Relevant Steady State Spectroscopic Data (In cm-1 to the Nearest 100 cm-1) for Ligation of ZnTPP (5 µM) by Pyrazine and Bipyridine in Aerated Toluene at 295 K absorption/1000 cm-1
Experimental Section Materials. 5,10,15,20-Tetraphenylporphinato zinc (ZnTPP, chlorin free), pyrazine and 4,4′-bipyridine were purchased from Aldrich and were used as received. Toluene (HPLC > 99.9%, Aldrich) was dried over molecular sieves. Instrumentation. Steady state UV-visible absorption experiments were carried out using a Varian-Cary 500 spectrophotometer, and steady state emission experiments were made with a PTI QuantaMaster spectrofluorometer. Emission spectra were corrected for variations in instrument detector sensitivity using the manufacturer’s correction data. Corrections due to fluorescence reabsorption in the Soret region were minimized by using a 10 mm × 2 mm cell and observing emission in the short path direction. S1-S0 fluorescence decays were measured by time-correlated single photon counting (TCSPC) using instrumentation and methods described in detail previously.14 Briefly, picosecond pulses from a diode-pumped, mode-locked, plane-polarized, pulse-picked, frequency-doubled Ti:sapphire laser (Coherent) were used to excite samples in the Soret region (typically at 400-430 nm). Following efficient internal conversion, fluorescence from S1 was observed at the magic angle via a monochromator and was detected by single photon counting with a Hamamatsu MCP PMT. The instrument response function was obtained by observing scattering from a porphyrin-free sample and was characterized by a typical fwhm of 0 would yield values of ∆GET(Q) that are even more positive. The S1 state of ZnTPP has a nanosecond lifetime, and its electronic relaxation processes therefore proceed primarily from the thermalized state when excited at wavelengths near the origin of the Q-band. Excitation at wavelengths far to the blue of the Q-band origin raise the possibility of ET from a vibrationally “hot” S1 molecule,5a,13 but evidence of this has not been found here. Ligation with these aromatic amines has only a minor effect on the electronic relaxation processes initiated following Q-band excitation. (ii) For a vibrationally thermalized system, the value of ∆GET(S) is positive for pyridine, but negative for bipyridine and is zero for pyrazine (within experimental uncertainty), suggesting that Soret band excitation of the ZnTPP complexes of pyrazine or bipyridinesbut not pyridinescan result in electron transfer. Because all our experiments involve excitation to the blue of the Soret band maximum and because intermolecular vibrational relaxation is slower by at least an order of magnitude than the ET processes observed, some fraction of an additional 0.2 eV of vibrational energy could be used to facilitate the ET process in these Soret-excited systems. ET from vibrationally “hot” species is precedented in covalently tethered porphyrin-acceptor systems.5a,13 These data therefore support the proposition that ICT is the required additional fast radiationless decay process responsible for de-excitation of the Soret-excited pyrazine and bipyridine complexes. Note also that both the initial forward and the subsequent back ET steps occur on a time scale of e200 fs, which is much faster than molecular solvation
Self-Assembled Zn Porphyrin-Bidentate Diamine Complexes processes (except for those involving changes in polarizability) and further justifies the use of C ) 0 in the calculation of ∆GET(S). (iii) On the basis of literature data alone (Table 4), we would predict that DABCO could be problematic as a bidentate ligand for assembling metalloporphyrin arrays that are to be used with photon actuation, not because it would behave as an electron acceptor but because it could act as an electron donor when the metalloporphyrin is excited in either its Q or Soret bands. Finally we note that the results of the S2-S1 net quantum efficiency measurements showing that ηS2S1 remains close to 1 in both ZnTPP and its ligated complexes suggests that the fate of the “ion pair” formed by ET is very fast charge recombination on the S1 surface of the complex. This process is also unusual but is precedented both in covalently bound dyads13,27 and triads5a and in noncovalent complexes.28 Using femtosecond transient absorption spectroscopy, Wallin et al.5a have recently shown that Soret-band excitation of a covalently bound unsymmetric zinc porphyrin (ZnP) donor-naphthalene imide (NI) acceptor triad exhibits near quantitative S2-S1-S0 relaxation via a charge-separated ZnP+NI- state in a manner similar to that observed previously by Mataga, et al.13 in dyads. The ZnP+NI- species formed from the S2 porphyrin is vibrationally hot and decays on a sub-picosecond time scale similar to that observed here for the ZnTPP-bipyridine system. Similar charge recombination processes have also been observed recently by Hirakawa et al.27 in pyrenyl-substituted zinc porphyrin derivatives where ICT between the Soret-excited chromophore and the covalently linked pyrene moiety results in rapid, near quantitative population of the S1 state of the metalloporphyrin. Morandiera, Vauthey, and co-workers28 first reported relaxation behavior similar to that observed here in the noncovalently bound ZnTPP-acetophenone system in which the thermodynamics of ET from the donor S2 state of ZnTPP are also favorable. The present results differ from those of Morandiera et al.,28 however, because (i) no ground state complex is observed between ZnTPP and the acetophenone, and (ii) very high concentrations (i.e., similar to the concentration of solvent) were needed to effect bimolecular quenching of the S2 state by ET. Morandiera et al.28 also observed ZnTPP S2 quenching by interaction with electron donors such as trimethoxybenzenes similar to the ZnTPP-DABCO system described heresbut again not by a mechanism involving a preformed ground state complex. The electronic energy level diagrams for the ZnTPP-pyrazine and ZnTPP-bipyridine systems can thus be adequately represented by Figure 6. Note that placement of the energy of the ICT state between the S2 and S1 energies of the ligated metalloporphyrin is arbitrary, particularly in light of the strong possibility that vibrationally unrelaxed species are involved,5a but justified on the basis of the thermodynamic calculations described above. It is interesting to note that previous TD-DFT calculations for ZnTPP and other d0 or d10 metalloporphyrins indicate that a large number of triplet states reside in this same energy range.21 The results of the present experimentssespecially those showing that ηS2S1 remains close to 1 in both ZnTPP and its ligated complexessindicate that these states (or doublet and quartet excited states derived from them by ET) do not participate in the excited state relaxation of these ligated molecules when excited in their Soret bands. Conclusions The bidentate ligands pyrazine and 4,4′-bipyridine are commonly employed in the self-assembly of supramolecular ag-
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Figure 6. Energy level diagram for ZnTPP-L ligated with the bidentate ligands pyrazine or bipyridine. The dotted arrows show the preferred path of relaxation following excitation (solid arrow) to the Soret state. The shaded area represents energies at which at least eight different excited triplet states lie but do not participate in the relaxation processes.
gregates containing metalloporphyrins that can be employed in a variety of photon-actuated devices. The results of steady state and time-resolved experiments using ZnTPP as a model metalloporphyrin show that complexation with these ligands dramatically alters the dynamics of such species when excited in the Soret region, but not the Q bands. Ligation of ZnTPP with either pyrazine or bipyridine provides an additional, unusual excited state relaxation path when the complex is excited in its Soret band; analysis based on a Rehm-Weller model demonstrates that this new path involves ultrafast electron transfer to an intramolecular charge transfer state lying between the S2 and S1 states of the complex. Initial population of the ICT state from S2 is followed by near quantitative back electron transfer to the S1 state. Application of the same model to systems involving other common bidentate ligands such as DABCO or multidentate nitrogen-containing ligands suggests that similar processes could be introduced in many other self-assembled metalloporphyrin systems. Acknowledgment. The authors gratefully acknowledge the continuing financial support of this research by the Natural Sciences and Engineering Research Council of Canada through Discovery and RTI grants to I. J. B. and R. P. S. Use of the facilities of the Saskatchewan Structural Sciences Centre and the technical assistance of Dr. Sophie Brunet are gratefully acknowledged. We thank Dr. Hans-Christian Becker of Chalmers University of Technology, Sweden, for use of his fitting program. References and Notes (1) (a) Yang, J.; Cho, S.; Yoo, H.; Park, J.; Li, W.-S.; Kim, D. J. Phys. Chem. A 2008, 112, 6869. (b) Osswald, P.; You, C.-C.; Stepanenko, V.; Wu¨rthner, F. Chem.sEur. J. 2010, 16, 2386. (c) Flamigni, L.; Talarico, A. M.; Ventura, B.; Marconi, G.; Sooambar, C.; Solladie´, N. Eur. J. Inorg. Chem. 2004, 2557. (2) (a) Brettar, J.; Gisselbrecht, J.-P.; Gross, M.; Solladie´, N. Chem. Commun. 2001, 733. (b) Solladie´, N.; Bouatra, S.; Merkas, S.; Rein, R.; Roeser, J. J. Porphyrins Phthalocyanines 2005, 9, 779. (3) (a) Grozema, F. C.; Houaner-Rassin, C.; Prins, P.; Siebbeles, L. D. A.; Anderson, H. A. J. Am. Chem. Soc. 2007, 129, 13370. (b) Winters, M. U.; Dahlstedt, E.; Blades, H. E.; Wilson, C. J.; Frampton, M. J.; Anderson, H. L.; Albinsson, B. J. Am. Chem. Soc. 2007, 129, 4291. (c) Anderson, H. A. Inorg. Chem. 1994, 33, 972. (4) (a) Zhang, Y.; Yang, R.-H.; Liu, F.; Li, K.-A. Anal. Chem. 2004, 76, 7336. (b) Brinas, R. P.; Troxler, T.; Hochstrasser, R. M.; Vinogradov, S. A. J. Am. Chem. Soc. 2005, 127, 1530. (c) Matsui, J.; Sodeyama, T.; Saiki, Y.; Miyazawa, T.; Yamada, T.; Tamaki, K.; Murashima, T. Biosens. Bioelectron. 2009, 25, 635. (5) (a) Wallin, S.; Monnereau, C.; Blart, E.; Gankou, J.-R.; Odobel, F.; Hammarstro¨m, L. J. Phys. Chem. A 2010, 114, 1709. (b) Perez-Inestrosa, E.; Montenegro, J.-M.; Collado, D.; Suau, R.; Casado, J. J. Phys. Chem. C
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