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Q and Soret Band Photoexcitation of Isolated Palladium Porphyrin Tetraanions Leads to Delayed Emission of Non-thermal Electrons over Microsecond Timescales Patrick Jäger, Katrina Brendle, Ulrike Schwarz, Miriam Himmelsbach, Markus K. Armbruster, Karin Fink, Patrick Weis, and Manfred M. Kappes J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00407 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016
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The Journal of Physical Chemistry Letters
Q and Soret Band Photoexcitation of Isolated Palladium Porphyrin Tetraanions Leads to Delayed Emission of Non-thermal Electrons over Microsecond Timescales Patrick Jäger1, Katrina Brendle2, Ulrike Schwarz2, Miriam Himmelsbach2, Markus K. Armbruster2, Karin Fink1, Patrick Weis2*, and Manfred M. Kappes1,2* 1
Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Postfach 3630, 76021
Karlsruhe, Germany 2
Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2,
76131 Karlsruhe, Germany
KEYWORDS Porphyrins, PdTPPS, Multianions, Photoelectron spectroscopy, Photodissociation spectroscopy, Photodetachment spectroscopy, Mass spectrometry
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Abstract. We have used both action and photoelectron spectroscopy to study the response of isolated PdII meso-tetra(4-sulfonatophenyl)porphyrin tetraanions ([PdTPPS]4-) to electronic excitation over the 2.22 – 2.98 eV photon energy range. The action spectrum obtained by recording the wavelength dependent intensity of charged decay products closely resembles the absorption spectrum of PdTPPS in aqueous solution (which shows pronounced Q and Soret absorption bands). The two main decay channels observed are sulfonate group loss and predominantly - electron emission. To better understand the electron emission channel, we have also acquired photoelectron spectra at multiple detachment photon energies covering the range probed in action spectroscopy. Upon both Q and Soret band excitation, we find that electrons are emitted in three characteristic kinetic energy ranges. The corresponding detachment processes are identified as (delayed) tunneling emission from both excited singlet and triplet states (each of which is accessed by/after one photon absorption) as well as resonant two-photon detachment. The first triplet state lifetime of isolated [PdTPPS]4- is significantly longer than 10 µs, possibly on the 100 µs time scale. We estimate that more than 50% of the electron emission observed upon photoexcitation occurs by way of this triplet state.
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Porphyrins comprise the active centers in a wide range of biological systems such as hemoglobin and chlorophyll. Therefore they are well-studied in the condensed phase1. Their unique spectral properties have also led to a large number of applications, e.g. as light harvesting molecules in solar cells2,3. Furthermore, water soluble metal porphyrins play an important role as photosensitizers in photodynamic therapy4. In this respect, PdII meso-tetra(4sulfonatophenyl)porphyrin (PdTPPS) has attracted considerable interest due to its long triplet state lifetime5, its high singlet oxygen sensitization efficiency and the observation of efficient radical formation upon photoexcitation (of aqueous solutions comprising [PdTPPS]4- tetraanions) 6,7
. Compared to the large number of condensed phase studies, spectroscopic investigations of
porphyrins and related molecules in gas phase are still rather limited8,9,10,11,12,13. Such measurements, free of the oftentimes perturbing solvent environment, are useful to gain a more detailed understanding of the underlying fundamental photophysical properties of the (metallo)porphyrins and their aggregates14. Depletion spectroscopy, or the corresponding “action” measure of decay products with mass-to-charge ratios different from those of the photoexcited parents, has been extensively used to characterize the electronic absorption properties of large isolated ions. In particular multianionic chromophores15 can be sensitively probed by these methods.
Here we use a combination of action and photoelectron
spectroscopy to study [PdTPPS]4- tetraanions in isolation. Our results provide not only transition energies and relative cross sections but also yield new insight into the energy relaxation mechanism following electronic excitation of [PdTPPS]4-. In particular we find that in gas-phase a large fraction of the photoexcited multianions undergoes intersystem crossing to a long-lived triplet state which itself decays predominantly by tunneling electron detachment to yield the (radical) trianion. Action spectroscopy combining photodissociation and photodetachment channels (PD(A)S) was performed on a Bruker APEX II 7T FT-ICR mass spectrometer previously described in detail16,17. Briefly, mass selected ions held under ultrahigh vacuum conditions in the room temperature Penning ion trap were irradiated with a pulsed nanosecond laser. The charged decay products formed were analyzed by recording the resulting mass spectrum (ca. 1 ms later). Ions were transferred into the gas phase from solution by using a home built nano electrospray ionization (nano-ESI) source running at room temperature14. PdII meso-tetra-(4sulfonatophenyl)porphyrin-hydrochloride was obtained from Frontier Scientific and dissolved in methanol/water (2:1) in a concentration of 1 mmol/l. For PD(A)S in the UV/visible spectral range, a combination of a Nd:YAG pump laser (Powerlite 8020, Continuum, San Jose CA; USA)
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and optical parametric oscillator (Panther EX OPO, Continuum, San Jose CA; USA) was used. The OPO was pumped by the third harmonic of the Nd:YAG laser at 355 nm with a repetition rate of 20 Hz and pulse length of 5-7 ns, yielding a maximum pulse energy of 15 mJ at 430 nm. For the PD(A)S experiments reported here, the laser output was uniformly attenuated so as to yield action signals which depended essentially linearly on photon fluence throughout the 410650 nm wavelength range scanned - as checked by recording fluence dependencies at 420 (Soret band) and 525 nm (Q-band). A complete PD(A)S spectrum was obtained by scanning the OPO in steps of 0.03 nm/s from 410 nm to 650 nm leading to a total scanning time of about 130 min. For photoelectron spectroscopy (PES), a custom built instrument comprising an ion mobility drift tube, a quadrupole mass filter and a magnetic bottle time-of-flight photoelectron spectrometer was used, "IMS-MS-PES". Details of the setup and typical conditions used for PE-spectroscopy with fixed wavelength UV irradiation have been given elsewhere18. Electrospray ionization source conditions were similar to those described above for the FT-ICR setup. However a conventional ESI source was used instead of nano-ESI. Recently, the instrument has been upgraded with a second quadrupole mass filter mounted downstream of the photoelectron spectrometer stage in order to allow photodepletion/action spectroscopy of ions fractionated according to both collision cross section and mass-to-charge ratio (IMS-MS-PD(A) – see Fig. S1). In the experiments reported here, the apparatus was used to acquire photoelectron spectra at UV detachment wavelengths of 213 and 266 nm as provided by the fifth and fourth harmonics of a pulsed Nd:YAG laser (Spectra Physics, LAB150-30), respectively.
Additionally,
photoelectron spectra were recorded at multiple wavelengths over the 415-460 and 500-560 nm ranges (in steps of 5 nm and at constant photon flux). For this the tunable output of a second Nd:YAG pumped optical parametric oscillator was used (Panther EX OPO, Continuum, San Jose CA; USA) with specifications much like the FT-ICR laser system described above. Two different PES measurement configurations were used (see figure S1). In the "parallel" configuration, the detachment laser was aligned collinear with (but counterpropagating to) the ion beam, thereby irradiating an ion package that was several tens of microseconds long. In the standard "perpendicular" configuration, the detachment laser beam crossed the ion beam at right angles within the detachment region of the photoelectron spectrometer. Due to the small interaction volume (
