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J. Phys. Chem. B 2006, 110, 24354-24360
Excited State Dynamics and Nonlinear Absorption of a Pyrazinoporphyrazine Macrocycle Carrying Externally Appended Pyridine Rings Massimo Villano,† Vincenzo Amendola,† Giancarlo Sandona` ,† Maria Pia Donzello,‡ Claudio Ercolani,*,‡ and Moreno Meneghetti*,† Department of Chemical Sciences, UniVersity of PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy, and Department of Chemistry, UniVersity of Roma “La Sapienza”, P.le Aldo Moro 5, I-00185 Roma, Italy ReceiVed: July 26, 2006
The multiphoton absorption properties of the tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato(monoacquo)Mg(II) complex [Py8TPyzPzMg(H2O)] (1) are reported and interpreted. The nonlinear optical behavior of 1 and the characterization of the excited states important for the nonlinear absorption process were studied at the pump frequency of the second harmonic generation of a Nd:YAG laser in the nanosecond time regime. It was found that complex 1 shows a very good optical limiting performance at 532 nm, which derives from two processes: (a) a reverse-saturable absorption process, which involves a triplet excited state at low intensities, and (b) a two-photon absorption process at higher intensities, which is due to the formation of the radical monoanion of 1, [Py8TPyzPzMg(H2O)]•-, during the photoreduction of the triplet state. The participation of a monoanion in determining the overall nonlinear absorption behavior of 1 is found, for the first time, for a tetrapyrrolic system. One can deduce that the involvement of the monoanion derives from the electronwithdrawing effect of the dipyridinopyrazino fragments externally attached to the porphyrazine core which make the reduced form of 1 easily accessible. These results suggest a modification of tetrapyrrolic systems with new nonlinear absorption properties.
Introduction Molecular materials with optical limiting (OL) properties have been extensively studied for their potential applications in protecting eyes and light-sensitive elements against accidental or hostile intense radiation.1 Several types of OL mechanisms are known, and among them, one can recall nonlinear refraction, free-carrier absorption, two-photon absorption, and reverse saturable absorption (RSA). The RSA mechanism occurs in molecules with an excited-state absorption cross section (σex) which exceeds that of the ground state (σg), and it has received the most attention in the context of organic chromophores.1 Highly π-delocalized systems show, in many cases, an RSA behavior that has been reported for phthalocyanines,2 porphyrins,3 fullerenes,4 carbon nanotubes,5 polydiacetylenes,6 and some organometallic compounds.7 In particular, phthalocyanines and porphyrins are especially attractive because of their relatively low linear absorption and high excited-state absorption cross section in the 450-600 nm region.8 The introduction of external substituents on the phthalocyanine macrocycle allows for the tailoring of these conjugated macrocycles in a plethora of ways, offering a wide possibility for the modification of their multiphoton absorption properties.2a-2d Porphyrazines, that is, phthalocyanine analogues, carrying heterocyclic rings, such as pyridine and pyrazine rings, annulated to the pyrrole rings of the porphyrazine core have also been synthesized and characterized.9 Their physicochemical properties appear to be strongly influenced by the presence of external N atoms, which determines, for instance, the formation * To whom correspondence should be addressed.
[email protected];
[email protected]. † University of Padova. ‡ University of Roma “La Sapienza”.
E-mail:
of stable hydrates.9b The presence of external heterocyclic rings containing different heteroatoms (N, S, Se) remarkably affects the electron deficiency of the entire macrocycle, leading to stepwise one-electron reductions, which occur at significantly less negative potentials than those of the parallel phthalocyanine species, as recently shown for diazepinoporphyrazines,10a pyrazinoporphyrazines,10b-e and thiadiazoloporpyrazines.10f Excited-state spectra of phthalocyanines are well known and show, in the nanosecond to microsecond time scales, a broad absorption in the visible spectral region approximately located in the spectral range between the B and Q bands.2e,2f The spectrum has been recognized as that of the triplet state of the molecules, which gives the most important contribution to the RSA behavior of these systems. Only a limited number of studies about the excited state properties of porphyrazines and their OL behavior are present in the literature.11 This has led us to evaluate the OL properties of a pyrazinoporphyrazine macrocycle, the tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato(monoaquo)-Mg(II), [Py8TPyzPzMg(H2O)] (1, Scheme 1), in DMSO at 532 nm and to measure its excited-state spectrum. The excited-state absorption spectrum of 1 has been determined by exciting the macrocycle with nanosecond laser pulses at 532 nm, both at low and high intensities. We have found that the RSA mechanism, which is common for this type of macrocycle, is not the exclusive process responsible for the OL of 1 and that its one-electron reduced form has an important role in the observed nonlinear absorption behavior, particularly at high intensities. For the interpretation of the nonlinear transmission experimental data, a model will be presented that shows the importance of the excited states of both the neutral molecule and its reduced form. Previously, only the OL data of 1 were presented for a comparison with other metal
10.1021/jp0647683 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/11/2006
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SCHEME 1: Structure of the Complex [Py8TPyzPzM] (M ) Mg(H2O); 1)
Figure 1. UV-vis spectrum of 1 in DMSO.
complexes of the same molecule,10c and we discussed the role of the possible aggregation of the molecules in solution. The importance of the photoreduced state in determining the multiphoton absorption processes of porphyrazines was not reported, to our knowledge, in other studies, which shows the importance of the substituents of the porphyrazine skeleton in obtaining new nonlinear optical behaviors. Experimental Section Complex 1 was synthesized as described elsewhere.10b DMSO solutions of the complex (∼0.1 mM) were used for the OL experiments. Nonlinear transmission measurements (NLT) were carried out with 9 ns pulses of a Nd:YAG laser (Quantel YG980E) at its duplicate frequency (532 nm). Incident and transmitted energies were measured pulse by pulse with a calibrated photodiode and a pyroelectric detector (Scientech SPHD25), respectively, at 2 Hz in an open-aperture (collecting solid angle is 0.04 str) configuration. The fluence of the incident pulses was controlled with a λ/2 wave plate and a polarizing cube-beam splitter. The pulse area on the sample was about 0.050 cm2, and the fluence (F) of the incident beam was varied in the range 10 e F e 3000 mJ cm-2, corresponding to 2.98 1024e I e 8.93 1026 ph cm-2 s-1. Linear optical spectra, recorded with a UV-vis-NIR spectrometer (Varian Cary 5) before and after the nonlinear transmission measurements, showed the absence of detectable decomposition of the macrocyclic species during the experiments. In the pump and probe experiments, the sample was excited at different fluences with the same pulses used for the NLT measurements. The white light of a Xe lamp was used as a probe. A digital oscilloscope (1 GHz, Le Croy LC564A) recorded the transient signals observed with a Jobin Yvon Horiba TRIAX 320 spectrometer, equipped with a Hamamatsu phototube (R2257), in the range 400-600 nm, which is the spectral region where the excited state absorption of this kind of tetraazaporphyrin macrocycle is usually observed. A notch filter was used to remove the exciting laser line from the probe signal analyzed by the spectrometer. A total of 200-300 shots were averaged to minimize the signal to noise ratio. Electrochemical reduction of the molecule was obtained with an EG&G model 273A potentiostat-galvanostat. Cyclic voltammetry and macroscale electrolysis experiments were performed with a classical three-electrode setup. Electrochemical experiments were performed in DMSO solutions containing 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte, using high purity argon to deoxygenate the solution. Cyclic voltammograms obtained on a saturated solution of the complex
at a scan rate of 100 mV s-1 indicate a behavior similar to that previously observed with a pyridine solution.10c Exhaustive macroscale electrolysis was then performed, with stirring, on a 10 mL DMSO solution containing 2.3 mg of 1 at a potential of 100 mV more negative than the peak potential of the first reduction peak. Electrolysis was stopped when the measured current was less than 10% of the initial value. Results The UV-vis spectrum of the porphyrazine complex 1 in DMSO (Figure 1) closely resembles that normally observed for porphyrazine9 and phthalocyanine macrocycles,12 with a Q band in the 600-700 nm spectral region, accompanied by vibronic satellites at shorter wavelengths, and a B band in the near UV region (350-400 nm), both of which are due to π f π* transitions.12,13 The sharp and unsplit Q band is indicative of a D4h symmetry, as expected for the macrocycle with the central Mg(II). The narrow shape of the band also indicates that the complex is essentially in its monomeric form. This is an important aspect since aggregation phenomena, which can play an important role in OL behavior by modifying, in particular, excited state relaxations, have been frequently observed for the entire class of pyridinopyrazinoporphyrazines, either as freebase ligand10b or metal derivatives,10c depending on the particular species and solvent utilized. The nonlinear transmission behavior of a 0.15 mM solution of 1 in DMSO with linear transmission T0 ) 87% at 532 nm (532 ) 1.718 × 103 M-1 cm-1) using 9 ns laser pulses is shown in Figure 2. This figure shows the transmittance vs input fluence (Fin) in a logarithmic scale (or the corresponding intensities (Iin); Figure 2a) and output fluences (Fout) vs Fin (or Iin; Figure 2b), emphasizing, with the former, the low fluence region and, with the latter, that at high fluence. As shown in Figure 2a, nonlinear behavior is already observed at low incident fluences (∼20 mJ cm-2). Two different slopes are found at low and high fluences, which are separated by a plateau in the 240-380 mJ cm-2 range. The solution is found to be stable up to fluences of about 2200 mJ cm-2, as confirmed by the UV-vis spectrum, which appears unchanged after the nonlinear optical measurements. This is a relevant aspect since other phthalocyanine analogues can undergo some decomposition under comparable experimental conditions of irradiation.14 The nonlinear transmission behavior of phthalocyanines usually derives from the absorption of an excited triplet state, which is populated by intersystem crossing from an excited singlet state. In these cases, the NLT curves are characterized by a low nonlinear threshold and by a smooth decrease in transmittance against fluence. The experimental data reported in Figure 2 show a low NL threshold, but as pointed out before, one cannot see a smooth behavior. For this reason, we measured pump and probe spectra to characterize the spectrum of the
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Figure 2. Nonlinear optical transmission of a 0.15 mM solution of 1 in DMSO at 532 nm.
Figure 3. (a) Transient excited spectra of 1 at F ) 200 mJ cm-2 after 0.5 µs (black line), 10 µs (red line), and 80 µs (green line) from the excitation pulse. (b) Temporal traces of the optical absorption of 1 in DMSO at 450 nm (black line) and 500 nm (red line) following photoexcitation at 532 nm.
Figure 4. Transient excited spectra of 1 obtained at F ) 400 mJ cm-2 (a) and F ) 30 mJ cm-2 (b) after 25 ns (black line) and 100 ns (red line) from the excitation pulse.
excited states involved in the nonlinear behavior. Pump and probe experiments were performed on a DMSO solution of 1, probing at different wavelengths and pumping with various fluences at 532 nm. The excited-state spectra obtained with an incident fluence of 200 mJ cm-2 are reported in Figure 3a. They are characterized by two main absorption peaks located at about 450 and 560 nm, whereas photobleaching can be observed in the spectral range below 410 nm and above 590 nm. The structure of the spectrum does not evolve with time and is found to be different from that which is usually observed for the transient spectrum of phthalocyanines, which shows one main peak at about 500 nm that is due to the absorption of the triplet state. Two typical temporal traces of the transient optical absorption of 1 at 450 and 500 nm, following excitation at 532 nm, are reported in Figure 3b. One observes, after the pulse, a transient absorption that lasts several tens of microseconds. In Figure 3a, one can
also see that the spectrum is very similar at 0.5 µs and at 80 µs after the pulse. The influence of the laser beam fluences on the excited-state absorption of 1 was studied performing pump and probe experiments at low (30 mJ cm-2) and high (400 mJ cm-2) fluences, where the two regimes of the NLT data can be better recognized. Furthermore, we analyzed the temporal traces immediately after excitation at 532 nm to obtain a more clear view of the evolution of the excited spectra absorption. Figure 4 reports excited-state spectra of 1 at 25 and 100 ns after the excitation pulse. At higher fluences, two absorption maxima at about 450 and 560 nm can be observed, as seen in Figure 3; however, in this case, a larger relative intensity of the 560 nm peak with respect to that of the peak at 450 nm is observed. A quite different behavior is observed at low fluences (Figure 4b). Immediately after the excitation pulse (25 ns), the two absorption peaks
Multiphoton Absorption of a Porphyrazine Complex
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SCHEME 2: Schematic Energy Level Diagram for the Triplet Absorption and Photoinduced Electron Transfer (ET) Process of [Py8TPyzPzMg(H2O)] upon 532 nm Excitationa
a
Blue lines and red lines refer to electronic states of the neutral and radical anion of 1, respectively.
previously observed at larger fluences are not present. Rather, the spectrum shows a broad absorption with a maximum at about 470 nm. However, at 100 ns after excitation, the spectrum becomes more similar to that found at higher fluences with two peaks at about 450 and 560 nm. The above data clearly suggest that two species are present in the observed processes: one that is widely prevalent at low fluences and a second that is mostly present at higher fluences. While the spectrum at low fluences strongly resembles that of the triplet species commonly seen for phthalocyanines, the spectrum observed at high fluences must be assigned to the radical anion ([Py8TPyzPzMg(H2O)]•-), as unequivocally indicated by spectroelectrochemical measurements, which show that, in the spectral region between 400 and 600 nm, the monoanion has two characteristic bands with maxima at 454 and 566 nm.10c Discussion and Model Interpretation A multiphoton absorption based on a reverse saturable absorption (RSA) mechanism that involves triplet excited states is known to be the active process for structures such as porphyrins, phthalocyanines, and porphyrazines.2e,2f,11a,14,15 Our pump and probe data suggest that this is also the case for the present species, [Py8TPyzPzMg(H2O)] (1), at low fluences. However, at high fluences, a charged molecular system, namely, the monoanion [Py8TPyzPzMg(H2O)]•-, is the widely dominant species. To interpret the above data, we developed a model which accounts both for the presence of the triplet state of 1 at low fluences and for that of the monoanion at higher fluences. A different oxidation state, induced by a photoexcitation process, was already observed in other cases, and usually this state is generated from a long-lived state like a triplet state.16 Since the presence of a triplet state, in our case, is observed at low fluences, our model must take into account an intersystem crossing from an excited singlet state to a triplet manifold and the formation of the monoanion from the triplet state. However, if we consider that the monoanion is formed from the lowest state of the triplet manifold, we are not able to fit the experimental data. The situation is different if we consider that
the monoanion is produced when the molecule is in the first excited state of the triplet manifold. Scheme 2 shows the energy level diagram and the relevant parameters for the dynamics. In this case, we find a good fit of the experimental data by taking into account a two-photon absorption for the monoanion since a one-photon excitation does not produce the steep variation in the transmittance at high fluences, as found in the experimental data. The set of differential equations describing the dynamics of the ground- and excited-state populations can be written according to a general scheme17 as follows:
∂N1 ) σ(1) g I-(N2 - N1) + k1N2 + k3N3 + k6N5 ∂t ∂N2 ) σ(1) g I-(N1 - N2) - k1N2 - kISCN2 ∂t ∂N3 ) σ(1) ex I-(N4 - N3) + k2N4 + kISCN2 - k3N3 ∂t ∂N4 ) σ(1) ex I-(N3 - N4) - k2N4 - kETN4 ∂t ∂N5 ) σ(1) gAI-(N6 - N5) + k5N6 - k6N5 ∂t ∂N6 ) ∂t (1) σ(2) ex I-(N7 - N6) + σgAI-(N5 - N6) - k5N6 + k4N7 + kETN4 ∂N7 ) σ(2) ex I-(N6 - N7) - k4N7 ∂t where Ni are the populations of the states and I- is the incident fluence. The solution of this set of equations18 gives the populations during the laser pulse, and therefore, one can calculate the transmitted intensity of the sample. Figure 5 shows
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Figure 5. Fitting of the experimental nonlinear transmission data of a 0.15 mM DMSO solution of 1 (continuous red lines).
Figure 6. Nonlinear optical transmission measurement of a solution of the monoanion [Py8TPyzPzMg(H2O)]•- at 532 nm.
Figure 7. Nonlinear optical transmission at 532 nm of a solution of 1 in DMSO with a linear transmittance of T0 ) 75%.
the good fit obtained by using the values of the fitting parameters that are reported in Table 1. The fitting results are quite good at both low and high fluences as shown in Figure 5. One finds that the important excitedstate absorption at low intensities is the triplet-state absorption (RSA process), whereas at high intensities the two-photon absorption (TPA process) of the monoanion is the relevant process. One should note that the cross section for the two-photon absorption from the excited state (σ(2) ex ) of the monoanion is very large, since it is four to five orders of magnitude larger than that usually observed for this type of nonlinear absorption when the initial state is a ground state. However, cross sections with such large values are known to be possible for excited states since they are very polarizable.19
The behavior of the monoanion, obtained by an electrochemical reduction at controlled potential in DMSO, was also examined by NLT measurements. The NLT experimental data of this solution at 532 nm are presented in Figure 6. The fit of the curve can be obtained by a simple two-photon absorption from the first excited state using the same levels for the radical anion reported in Scheme 2. The values of the fitting parameters are reported in Table 2, where one can note the same value for the two-photon absorption cross section (σex ) 6.25 × 10-42 cm4 ph-1 mol-1) and very similar parameters for the other processes. This result is of further support to the model proposed here (Scheme 2) for the dynamics of the neutral molecule. In many experiments reported in the literature, the concentration of the solutions used for the measurements is high and the linear transmission at 532 nm is low. In this situation, it is
Multiphoton Absorption of a Porphyrazine Complex TABLE 1: Values of the Fitting Parameters for Nonlinear Absorptions of 1 σg(1) [cm2] k1 [s-1] τ1 [ns] kISC [s-1] τISC [ns] σex(1) [cm2] k3 [s-1] τ3 [µs] k2 [s-1] τ2 [ps]
6.57 × 10-18 1.49 × 109 0.67 8.00 × 109 0.12 3.60 × 10-17 5.00 × 105 2.0 1.10 × 1010 90
kET [s-1] τET [ns] σex(2) [cm4 ph-1 mol-1] k4 [s-1] τ4 [ps] σgA(1) [cm2] k5 [s-1] τ5 [ps] k6 [s-1] τ6 [µs]
3.02 × 108 3.3 6.25 × 10-42 1.50 × 1012 0.66 4.74 × 10-17 1.00 × 1011 10 3.30 × 104 30
TABLE 2: Values of the Fitting Parameters for the Nonlinear Absorption Process of the Monoanion [Py8TPyzPzMg(H2O)]•σgA(1) [cm2] 3.87 × 10-17 k5 [s-1] 2.00 × 1011 τ5 [ns] 0.5
σex(2) [cm4 ph-1 mol-1] 6.25 × 10-42 k4 [s-1] 1.5 × 1012 τ4 [ps] 0.66
difficult to observe the processes which are activated at high intensities because of the prevalence of those activated at low intensities. We have observed this situation in the present case. In Figure 7, we report the NLT of a concentrated solution of complex 1 with a linear transmission of T0 ) 75% at 532 nm. In this case, the plateau observed for the low-concentration solution is not present and the most important contribution to the nonlinear absorption of 1 comes from the triplet excited state absorption. In fact, a good fit can be obtained by using the same parameters reported in Table 1, changing only the concentration of the solution. One can see that, by increasing the concentration, the contribution from the monoanion is reduced to the very high-intensity region above 900-1000 mJ cm-2. This shows that, at high concentrations, the RSA mechanism is the most important process responsible for the observed nonlinear response of 1, in line with the expectation for this kind of molecule; however, the presence of the monoanion must be considered for a more complete description of its nonlinear behavior, particularly at higher intensities. Conclusions The nonlinear optical properties of the Mg(II) complex of tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine ([Py8TPyzPzMg(H2O)]; 1) in DMSO solution have been studied in the nanosecond time regime at 532 nm. It has been found that the solution shows a very good optical limiting performance up to fluences of 2200 mJ cm-2. Moreover, the molecule shows very good photostability against possible light-induced decomposition, which is probably related to its larger stability against oxidation. Pump and probe measurements have shown that a multiphoton absorption based on a reverse saturable absorption (RSA) mechanism involving triplet excited states is the active process only at low fluences, similar to the case of phthalocyanines, whereas another molecular species, namely, the monoanion of 1, [Py8TPyzPzMg(H2O)]•-, dominates the situation at high fluences. At the molecular level, the nonlinear absorption process can be understood in terms of two sequential processes: (a) a simple four-level RSA mechanism starting from the neutral species 1, which is active at low intensities, and (b) a two-photon absorption from an excited state of the monoanion generated from the excited triplet state, which becomes active at high intensities. The results obtained with [Py8TPyzPzMg(H2O)] suggest a way to modify the tetrapyrrolic system, which shows nonlinear absorption properties depending not only on the triplet excited
J. Phys. Chem. B, Vol. 110, No. 48, 2006 24359 states but also on charged states. This can be particularly interesting in the nanosecond time regime for which long-lived excited states are important. Acknowledgment. Financial support by the Universities of Padova and Rome “La Sapienza” and by the MIUR (PRIN 200303804) is gratefully acknowledged. G. Marcolongo, S. Crivellaro, and C. Bergami are gratefully acknowledged for experimental help and useful suggestions. References and Notes (1) Tutt, L.; Boggess, T. F. Prog. Quantum Electron. 1993, 17, 299. (2) (a) Dini, D.; Hanack, M. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2003; Vol. 17, Chapter 107, pp 1-36. (b) De la Torre, G.; Va´zquez, P.; Agullo´Lopez, F.; Torres, T. Chem. ReV. 2004, 104, 3723-3750. (c) O’Flaherty, S. M.; Hold, S. V.; Cook, M. J.; Torres, T.; Chen, Y.; Hanack, M.; Blau, W. J. AdV. Mater. 2003, 15, 19-32. (d) Calvete, M.; Yang, G. Y.; Hanack, M. Synth. Met. 2004, 141, 231-243. (e) Perry, J. W.; Mansour, K.; Lee, I.-Y. S.; Wu, X.-L.; Bedworth, P. V.; Chen, C.-T.; Ng, D.; Marder, S. R.; Miles, P.; Wada, T.; Tian, M.; Sasabe, H. Science 1996, 273, 1533. (f) Shirk, J. S.; Pong, R. G. S.; Flom, S. R.; Heckmann, H.; Hanack, M. J. Phys. Chem. A 2000, 104, 1438. (g) Wen, T. C.; Lian, I. D. Synth. Met. 1996, 83, 111. (h) Perry, J. W.; Mansour, K.; Marder, S. R.; Perry, K. J.; Alvarez, D.; Choong, I. Opt. Lett. 1994, 19, 625. (i) Wie, T. H.; Hagan, D. J.; Sence, M. J.; Van Stryland, E. W.; Perry, J. W.; Coulter, D. R. Appl. Phys. B 1992, 54, 46. (3) Chou, J.-H.; Nalwa, H. S.; Kosal, M. E.; Rakov, N. A.; Suslick, K. S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 6, Chapter 41, pp 43131. (4) (a) Tutt, L. W.; Kost, A. Nature 1992, 356, 225. (b) Cha, M.; Sariciftci, N. S.; Heeger, A. J.; Hummelen, J. C.; Wudl, F. Appl. Phys. Lett. 1995, 67, 3850. (c) Signorini, R.; Zerbetto, M.; Meneghetti, M.; Bozio, R.; Maggini, M.; De Faveri, C.; Prato, M.; Scorrano, G. Chem. Commun. 1996, 1891. (d) Sun, Y. P.; Riggs, J. E.; Lin, B. Chem. Mater. 1997, 9, 1268. (5) (a) Tang, B. Z.; Xu, H. Y. Macromolecules 1999, 32, 2569. (b) Riggs, J. E.; Walker, D. B.; Carroll, D. L.; Sun, Y. J. Phys. Chem. B 2000, 104, 7071. (c) O’Flaherty, S. M.; Murphy, R.; Hold, S. V.; Cadek, M.; Coleman, J. N.; Blau, W. J. J. Phys. Chem. B 2003, 107, 958. (6) (a) Zhu, P.; Yu, C.; Liu, J.; Song, Y.; Li, C. Proc. SPIE 1996, 2897, 289. (b) Yoshino, F.; Polyakov, S.; Stegeman, G. I. Appl. Phys. Lett. 2004, 84, 5362. (7) (a) Zhang, C.; Song, Y. L.; Jin, G.; Fang, G.; Wang, Y. X.; Raj, S. S. S.; Fun, H. K.; Xin, X. Q. J. Chem. Soc., Dalton Trans. 2000, 1317. (b) Allan, G. R.; Labergerie, D. R.; Rychnovsky, S. J.; Bogess, T. F.; Smirl, A. L.; Tutt, L. J. Phys. Chem. 1992, 96, 6313. (8) (a) Edwards, L.; Gouterman, M. J. Mol. Spectrosc. 1970, 33, 292. (b) Shirk, J. M. In PhthalocyaninessProperties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publishers: New York, 1996; Vol. 4, pp 79-181. (9) (a) Stuzhin, P. A.; Ercolani, C. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2003; Vol. 15, Chapter 101, pp 263-364. (b) Kudrevich, S. V.; van Lier, J. E. Coord. Chem. ReV. 1996, 156, 163. (10) (a) Donzello, M. P.; Dini, D.; D’Arcangelo, G.; Zhan, R.; Ou, Z.; Ercolani, C.; Stuzhin, P. A.; Kadish, K. M. J. Am. Chem. Soc. 2003, 125, 14190. (b) Donzello, M. P.; Ou, Z.; Monacelli, F.; Ricciardi, G.; Rizzoli, C.; Ercolani, C.; Kadish, K. M. Inorg. Chem. 2004, 43, 8626. (c) Donzello, M. P.; Ou, Z.; Dini, D.; Meneghetti, M.; Ercolani, C.; Kadish, K. M. Inorg. Chem. 2004, 43, 8637. (d) Bergami, C.; Donzello, M. P.; Ercolani, C.; Kadish, K. M.; Monacelli, F.; Rizzoli, C. Inorg. Chem. 2005, 44, 9852. (e) Bergami, C.; Donzello, M. P.; Monacelli, F.; Ercolani, C.; Kadish, K. M. Inorg. Chem. 2005, 44, 9862. (f) Donzello, M. P.; Agostinetto, R.; Ivanova, S. S.; Fujimori, M.; Suzuki, Y.; Yoshikawa, H.; Awaga, K.; Ercolani, C.; Kadish, K. M.; Shen, J.; Stuzhin, P. A. Inorg. Chem. 2005, 44, 8539. (11) (a) Dini, D.; Hanack, M.; Meneghetti, M. J. Phys. Chem. B 2005, 109, 12691. (b) Hwang, L. C.; Tsai, C. Y.; Tiao, C. J.; Wen, T. C. Opt. Quantum Electron. 2000, 32, 641. (12) Mack, J.; Stillman, M. J. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2003; Vol. 16, Chapter 103, pp 43-116. (13) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. III, pp 1-165. (14) Dini, D.; Barthel, M.; Shneider, T.; Ottmar, M.; Verma, S.; Hanack, M. Solid State Ionics 2003, 165, 289. (15) Shirk, J. S.; Pong, R. G. S.; Bartoli, F. J.; Snow, A. W. Appl. Phys. Lett. 1993, 63, 1880.
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