Photoperoxidation of a Diamino Zinc Porphyrazine to the seco-Zinc Porphyrazine: Suicide or Murder?

We report on the efficient photooxidation of hexapropyl bis(dimethylamino) zinc porphyrazine. The process is shown to be autocatalytic. The triplet state of the seco-ZnPz sensitizes the formation of excited-state singlet oxygen with a quantum yield of 0.54 and subsequent cleavage of the pyrrole double bond occurs to give the product, seco-zinc porphyrazine. The photophysics of the two porphyrazines is examined using absorption, emission, and transient absorption spectroscopy. The efficiency of production of singlet oxygen is monitored using the phosphorescence emission signature at 1270 nm. Introduction We reported recently the synthesis of porphyrazineoctamines by macrocyclization of diamino maleonitrile derivatives using magnesium propoxide in propanol. 1 During several Linstead macrocyclization 2 reactions of bis(dimethylamino)maleonitrile, we observed the formation of a minor side product, the secoporphyrazine 1 In this paper, we report the results from the photophysical studies of both the reactant, 2, and product, 4, and show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product. As a consequence, the kinetics of the process are shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. Experimental Section Chemicals. The preparation, purification and characterization of ZnPz and seco-ZnPz have been described previously. Steady-State Absorption and Emission Measurements. Electronic absorption spectra were recorded on a dual beam UV/ vis spectrometer (Perkin-Elmer Lambda-2) with fixed 2 nm resolution. Fluorescence emission and excitation spectra were recorded on a spectrometer with xenon arc lamp excitation and a photon-counting detection system (Instruments SA Fluoromax). Fluorescence quantum yields were determined by the comparative method 7 using chlorophyll a in ether (φ F ) 0.32 ( 0.05) as the reference standard. To avoid unwanted reabsorption effects, all fluorescence measurements were recorded on solutions with Q-band absorbances of less than 0.1 in 1 cm path length cells. Time-Resolved Fluorescence Measurements. Fluorescence decays were recorded using a time-correlated single-photon counting spectrometer with pulsed laser excitation at an excitation wavelength, λ ex , of 670 nm, 10 ps pulse duration, and a repetition rate of 3.8 MHz. Fluorescence was detected perpendicular to the direction of excitation, dispersed through a subtractive dispersion monochromator, and subsequently detected by a microchannel plate photomultiplier tube that provided an overall time resolution of less than 100 ps. The decays were analyzed using a nonlinear, least-squares, iterative reconvolution procedure and stringent data fitting criteria. Results The electronic absorption spectrum of ZnPz 2 has a distinct Q-band at 600 nm (see A purified sample of ZnPz was dissolved in cyclohexane to a concentration of 1.07 × 10 -5 mol dm -3 , placed in a conventional 1 cm cuvette, and allowed to equilibrate with air at 295 K and an absorption spectrum was measured. The sample was irradiated with white light by placing it 30 cm in front of a 60 W tungsten filament lamp for 10 min. The sample was then returned to the spectrometer and a new spectrum was Photoperoxidation of a Diamino Zinc Porphyrazine J. Phys. Chem. A, Vol. 103, No. 22, 1999 4353 recorded. This process was then repeated until no further changes in the spectrum could be observed. The results from this study are shown in Before irradiation the emission shows one peak at 605 nm. A new peak at 708 nm grows in with irradiation, reaching a maximum after 20 min, as shown in the inset of Fluorescence decays of seco-ZnPz at various emission wavelengths were measured after irradiation and were adequately described by a biexponential function with decay times of 0.73 ns and 0.16 ns and contributions of 76.6% and 23.4%, respectively. A fresh sample of ZnPz in degassed toluene was prepared in order to record fluorescence emission and excitation spectra of the ZnPz, but no fluorescence was detected, although the 605 nm peak attributed to the octapropyl zinc porphyrazine was still evident. The fluorescence data for ZnPz and seco-ZnPz suggest that the major deactivation pathways for the excited state of each are nonradiative. The possibility that this is due to intersystem crossing to the triplet state was then investigated using kinetic and spectral transient absorption spectroscopy. Bleaching of the ground state population is clearly indicated by the depletion in the absorption cross section of the sample at 650 and 560 nm, corresponding to the Q-bands, and at 340 nm, corresponding to the Soret band. To this bleaching is added a broad triplet state (T 1 f T n ) absorption that is clearly observed in the region 390-520 nm, where little ground-state singlet (S 0 f S n ) absorption occurs. The extinction coefficient at the maximum in this range, 427 nm, was calculated to be 0.97 × 10 4 dm 3 mol -1 cm -1 and the quantum yield for triplet state formation, φ T , as 0.64 ( 0.06. A typical transient decay profile for air-free seco-ZnPz in this spectral region is shown in A sample of air-equilibrated seco-ZnPz in toluene solution was then investigated by observing the effect of the 3 O 2 on the transient absorption decay and also by detecting emission at 1270 nm, where singlet oxygen phosphoresces. The change in the transient decay rate of the seco-ZnPz was so dramatic as to be difficult to measure accurately. Conversely, characteristic singlet oxygen emission was readily observed indicating that the triplet state of the seco-ZnPz was efficiently quenched by the dissolved oxygen. The singlet oxygen emission decayed exponentially with a characteristic lifetime of 29 µs, as shown 4354 J. Phys. Chem. A, Vol. 103, No. 22, 1999 Montalban et al. in Analysis The absorption spectra, shown in parts a and j of The four molar decadic extinction coefficients are given in The functional form of these kinetic data is neither first nor second order, but is indicative of autocatalysis, where the product, seco-ZnPz, catalyzes the rate of photoperoxidation of the ZnPz. The rate law for the autocatalysis is given by (1) Photoperoxidation of a Diamino Zinc Porphyrazine J. Phys. Chem. A, Vol. 103, No. 22, 1999 4355 ZnPz] 0 ) 7 × 10 -9 mol dm -3 , and k ) 1.5 × 10 4 dm 3 mol -1 min -1 the data were satisfactorily modeled using eqs 4 and 5, the results of which are shown as the solid lines in To be consistent with the model, the concentration of singlet oxygen must be directly proportional to the concentration of seco-ZnPz such that the proportionality constant remains unchanged during the course of the reaction. These assumptions can be shown to be true (see appendix) if the following criteria are met. (1) Some minimal sensitization must first occur in order to initiate the photoperoxidation, but the rate of this process is rapidly exceeded by the seco-ZnPz. (2) The rate constant for the peroxidation process is very small with respect to the normal, unimolecular deactivation rate constants of the singlet oxygen. (3) The concentration of dissolved oxygen must be large with respect to the initial concentration of ZnPz, such that it remains effectively unchanged during the course of the photoperoxidation. In this case, the apparent second-order rate constant is expected to be proportional to the light intensity and related to the ground-state oxygen concentration (see appendix). The initial concentration of seco-ZnPz, [seco-ZnPz] 0 , was determined to be 7 × 10 -9 mol dm -3 , a value that is 3 orders of magnitude smaller than that of the ZnPz starting material. The experiment was repeated twice more with the starting solution spiked with different amounts of seco-ZnPz. In both cases, the overall rate of reaction increased dramatically, although the derived second-order rate constant, k, was found to have a value within 10% of the value for the unspiked reaction. It is thus evident that the sensitizing properties of the seco-ZnPz product far exceed those of the ZnPz starting material. This conclusion is consistent with the results from the triplet state studies in which the quantum yield for triplet state formation, φ T , is 0.64 for the seco-ZnPz but immeasurable for the ZnPz. The rapid growth of the singlet oxygen signal, when examining air-equilibrated solutions of ZnPz, can similarly be attributed to the formation of seco-ZnPz which then catalyzes further production. The singlet oxygen phosphorescence signal decays at the same rate during the course of the photoperoxidation, which demonstrates that the rate constant for this chemical reaction is small with respect to the rate constants for the other excited-state deactivation mechanisms, as required by the second criterion listed above, for the autocatalysis kinetic mechanism. The final requirement, that the concentration of normal, triplet state oxygen remains unchanged, is also fulfilled as the initial concentration of ZnPz was 1.07 × 10 -5 mol dm -3 while that of dissolved oxygen in toluene at room temperature 13 is 200 times higher at 2 × 10 -3 mol dm -3 . The photoperoxidation of ZnPz to seco-ZnPz results from the attack on ZnPz by singlet oxygen that is sensitized by the triplet state of the seco-ZnPz product (vide supra). The concentration of ground-state oxygen remains unchanged during the chemical reaction and the rate constant for the photoperoxidation process is much smaller than all other excited-state deactivation mechanisms for singlet oxygen. These effects lead to a concentration of the reactive, singlet oxygen that is proportional to the concentration of the product, seco-ZnPz (eq A5) and hence an autocatalytic kinetic mechanism is observed. The rate constant, k, is independent of the initial concentrations of both ZnPz and seco-ZnPz, although it does depend on the light intensity and on [ 3 O 2 ], but this was not explored fully. Using the same irradiation conditions, the effect of solvent was very pronounced with noncoordinating solvents, such as toluene, cyclohexane, and dichloromethane behaving in a similar fashion. The use of pyridine as a coordinating solvent reduced the rate constant for the reaction by a factor of 20. This is consistent with our previous observation 4 that ZnPz appeared to be more stable in coordinating solvents. Discussion The photophysical data for ZnPz suggests that the dominant deactivation process for the first excited singlet state is neither fluorescence nor intersystem crossing and by the process of elimination it is probably direct internal conversion to the ground state, followed by vibrational relaxation. The seco-ZnPz is more interesting photophysically, exhibiting fluorescence as well as intersystem crossing (φ T ) 0.64) as the dominant deactivation pathway. The sensitivity of fluorescence spectroscopy enables the radiative process to be observed readily even though it is 200 times less efficient. Again internal conversion probably constitutes the remaining 36% of the deactivation mechanisms. The emission peak at 605 nm we assign to a small amount of octapropyl zinc porphyrazine not observable by electronic absorption or other techniques. The emission at 708 nm from the seco-ZnPz has a very low fluorescence quantum yield. It decays biexponentially, but both of the measured fluorescence decay times are relatively long, results that are consistent with long natural, radiative lifetimes. As a guide, the average lifetime of the emission of 0.6 ns can be combined with quantum yield to produce a natural radiative lifetime of 170 ns. This value is inconsistent with the very large extinction coefficient ( 648 ) 38 320) for the lowest energy Q-band of the seco-ZnPz, where the Strickler-Berg relationship 14 predicts a value closer to 2 Phys. Chem. A, Vol. 103, No. 22, 1999 Montalban et al. ns. It could, however, be due to emission emanating from a state of the seco-ZnPz that is lower in energy than the Q-band at 648 nm, but is not coupled radiatively with the ground state and does not appear in the absorption spectrum. This argument is consistent with the large Stokes shift between the lowest energy absorption band (648 nm) and the emission maximum at 708 nm. The production of singlet oxygen by seco-ZnPz must occur via the triplet state as the excited singlet state of seco-ZnPz is too short lived to interact with ground-state oxygen by diffusion. The quantum yield of singlet oxygen production is a combination of the intersystem crossing and sensitization by quenching. Since ZnPz has a low intersystem crossing quantum yield, the difficulty in measuring a singlet oxygen signal for ZnPz may not be due to inefficiency in the sensitization by the triplet state. The peroxidation process is, however, initiated. This may be an indication that sensitization by ZnPz is possible, but not easily measured, or that the process is initiated by the extremely small amount of octapropyl zinc porphyrazine that fluoresces at 605 nm. The efficiency of seco-ZnPz in promoting the sensitization is due to high yields of both intersystem crossing and quenching. The quantum yields for the two processes, φ T ) 0.64 and φ ∆ ) 0.54, respectively, indicate that quenching of the triplet state of seco-ZnPz by ground-state oxygen is indeed the major deactivation pathway. The high quantum yield of triplet state formation could be promoted by the two carbonyl groups on the seco-ZnPz, as this group is well-known to promote intersystem crossing. 15 Conclusion The photoperoxidation of ZnPz proceeds via the attack of 1 O 2 on a peripheral pyrrole ring to produce seco-ZnPz. Once initiated, the reaction accelerates due to more efficient sensitization of the oxygen by the photoproduct, seco-ZnPz. As a consequence, the reaction proceeds autocatalytically with a second-order rate constant, the magnitude of which depends on the light intensity and on the concentration of dissolved oxygen. The photophysical characterization of the seco-ZnPz reveals a low fluorescence quantum yield and a high intersystem quantum yield. The triplet state lifetime is long (τ ) 81 µs in the absence of oxygen), allowing for efficient interaction with dissolved 3 O 2 to produce the active 1 O 2 , with a quantum yield of 0.54. This high value explains the efficiency of the photoperoxidation and also the autocatalytic mechanism. The use of coordinating solvents, such as DMF and pyridine, prevents the photooxidation process by inhibiting the efficiency of photosensitization by the seco-ZnPz. The photosensitized production of 1 O 2 by seco-ZnPz is very efficient which would make this dye extremely phototoxic and confirms the overall photooxidation as murder and not suicide. Acknowledgment

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