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Article
Ultrafast Charge Transfer in Nickel Phthalocyanine Probed by Femtosecond Raman-Induced Kerr Effect Spectroscopy Gurusamy Balakrishnan, Alexandra V. Soldatova, Philip J. Reid, and Thomas G. Spiro J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja503541v • Publication Date (Web): 19 May 2014 Downloaded from http://pubs.acs.org on May 27, 2014
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Journal of the American Chemical Society
Ultrafast Charge Transfer in Nickel Phthalocyanine Probed by Femtosecond Raman-Induced Kerr Effect Spectroscopy
Gurusamy Balakrishnan, Alexandra V. Soldatova, Philip J. Reid*, and Thomas G. Spiro*
Department of Chemistry, University of Washington, Seattle WA 98195
*Corresponding author’s email: [email protected] and [email protected]
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Journal of the American Chemical Society
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ABSTRACT The recently developed technique of femtosecond stimulated Raman spectroscopy, and its variant, femtosecond Raman-induced Kerr effect spectroscopy (FRIKES), offer access to ultrafast excited state dynamics via structurally-specific vibrational spectra. We have used FRIKES to study the photoexcitation dynamics of Ni(II) phthalocyanine with eight butoxy substituents, NiPc(OBu)8. NiPc(OBu)8 is reported to have a relatively long-lived LMCT state, an essential characteristic for efficient electron transfer in photocatalysis. Following photoexcitation, vibrational transitions in the FRIKES spectra, assignable to phthalocyanine ring modes, evolve on the fs-ps time scales. Correlation of ring core-size with the frequency of the ν10 (asymmetric C-N stretching) mode confirms the identity of the LMCT state, which has a ~500 ps lifetime, as well as that of a precursor d-d excited state. An even earlier (~0.2 ps) transient is observed and tentatively assigned to a higher lying Jahn-Teller-active LMCT state. This study illustrates the power of FRIKES spectroscopy in elucidating ultrafast molecular dynamics.
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Journal of the American Chemical Society
1. INTRODUCTION Phthalocyanines (Pcs) are of great interest in applications ranging from molecular electronics, photovoltaics, and photocatalysis, due to their favorable electronic structure and good thermal and chemical stability.1-5 Because of their intense and tunable light absorption in the red and near-infrared, they are useful as photodynamic and photothermal sensitizers for tumor therapy and other medical applications.6-8They also hold promise for dye-sensitized solar cells (DSSCs) given their ability to harvest a greater part of the solar spectrum than current DSSCs employing Ru(II)-bipyridine-based dyes.9-13 They could be used as co-sensitizers with dyes having complimentary absorption, in tandem solar cells, or via synthetic modification of the macrocycle structure to achieve panchromatic sensitization.14 Metallo-Pcs (MPcs) might also serve as useful photocatalysts, since axial coordination sites of the central metal are available for binding substrate molecules. If photoexcitation leads to ligand-to-metal or metal-to-ligand charge transfer states (LMCT or MLCT), then the transiently reduced or oxidized metal could reduce or oxidize bound substrate, provided that recombination were prevented by efficient discharge of the electron or hole on the phthalocyanine ring (for example, by attachment to a semiconductor electrode). The MPc-semiconductor conjugate could then be part of a DSSC, with built-in catalytic capability. The possibility of generating H2 by photo-reduction of water would be particularly appealing. A number of molecular water reduction catalysts are currently under investigation, but they need to work together with a separate absorber for solar applications.15-20 MPc’s offer the possibility of combining the absorber and catalyst in a single molecule. Realizing the photocatalytic potential of MPcs requires careful tuning of their excited states to ensure that charge separation is sufficiently long to sustain a catalytic reaction. Recently, Gunaratne et al.21 have identified a relatively long-lived excited state in NiPc(OBu)8, the Ni(II) complex of Pc with eight butoxy substituents (Figure 1). They assigned the excited state as a LMCT state from its transient absorption spectral characteristics and the expected energy level ordering from TDDFT calculations. The reported ~600 ps lifetime of the LMCT state might permit ring reduction at a p-type semiconductor electrode, making Ni(I)Pc(OBu)8 available for substrate reduction. NiPc(OBu)8 has the added attraction of a substantially red-shifted visible absorption band (Figure 1), increasing its usefulness as a solar absorber. Structural characterization of the excited states of a potential catalyst is valuable in elucidating the complex mechanism of catalytic transformations. In contrast to transient absorption spectra that are broad and contain little structural information, vibrational spectra typically demonstrate narrow transitions whose frequencies are highly sensitive to structure. Access to ultrafast time-resolved vibrational spectra has recently been facilitated by the development of various coherent Raman spectroscopy (CRS) techniques, such as, coherent anti-Stokes Raman spectroscopy (CARS) and femtosecond stimulated 3 ACS Paragon Plus Environment
Journal of the American Chemical Society
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Raman spectroscopy (FSRS), which are capable of providing Raman spectra with both high spectral (
Article
Ultrafast Charge Transfer in Nickel Phthalocyanine Probed by Femtosecond Raman-Induced Kerr Effect Spectroscopy Gurusamy Balakrishnan, Alexandra V. Soldatova, Philip J. Reid, and Thomas G. Spiro J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja503541v • Publication Date (Web): 19 May 2014 Downloaded from http://pubs.acs.org on May 27, 2014
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 42
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society
Ultrafast Charge Transfer in Nickel Phthalocyanine Probed by Femtosecond Raman-Induced Kerr Effect Spectroscopy
Gurusamy Balakrishnan, Alexandra V. Soldatova, Philip J. Reid*, and Thomas G. Spiro*
Department of Chemistry, University of Washington, Seattle WA 98195
*Corresponding author’s email: [email protected] and [email protected]
1 ACS Paragon Plus Environment
Journal of the American Chemical Society
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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ABSTRACT The recently developed technique of femtosecond stimulated Raman spectroscopy, and its variant, femtosecond Raman-induced Kerr effect spectroscopy (FRIKES), offer access to ultrafast excited state dynamics via structurally-specific vibrational spectra. We have used FRIKES to study the photoexcitation dynamics of Ni(II) phthalocyanine with eight butoxy substituents, NiPc(OBu)8. NiPc(OBu)8 is reported to have a relatively long-lived LMCT state, an essential characteristic for efficient electron transfer in photocatalysis. Following photoexcitation, vibrational transitions in the FRIKES spectra, assignable to phthalocyanine ring modes, evolve on the fs-ps time scales. Correlation of ring core-size with the frequency of the ν10 (asymmetric C-N stretching) mode confirms the identity of the LMCT state, which has a ~500 ps lifetime, as well as that of a precursor d-d excited state. An even earlier (~0.2 ps) transient is observed and tentatively assigned to a higher lying Jahn-Teller-active LMCT state. This study illustrates the power of FRIKES spectroscopy in elucidating ultrafast molecular dynamics.
2 ACS Paragon Plus Environment
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Journal of the American Chemical Society
1. INTRODUCTION Phthalocyanines (Pcs) are of great interest in applications ranging from molecular electronics, photovoltaics, and photocatalysis, due to their favorable electronic structure and good thermal and chemical stability.1-5 Because of their intense and tunable light absorption in the red and near-infrared, they are useful as photodynamic and photothermal sensitizers for tumor therapy and other medical applications.6-8They also hold promise for dye-sensitized solar cells (DSSCs) given their ability to harvest a greater part of the solar spectrum than current DSSCs employing Ru(II)-bipyridine-based dyes.9-13 They could be used as co-sensitizers with dyes having complimentary absorption, in tandem solar cells, or via synthetic modification of the macrocycle structure to achieve panchromatic sensitization.14 Metallo-Pcs (MPcs) might also serve as useful photocatalysts, since axial coordination sites of the central metal are available for binding substrate molecules. If photoexcitation leads to ligand-to-metal or metal-to-ligand charge transfer states (LMCT or MLCT), then the transiently reduced or oxidized metal could reduce or oxidize bound substrate, provided that recombination were prevented by efficient discharge of the electron or hole on the phthalocyanine ring (for example, by attachment to a semiconductor electrode). The MPc-semiconductor conjugate could then be part of a DSSC, with built-in catalytic capability. The possibility of generating H2 by photo-reduction of water would be particularly appealing. A number of molecular water reduction catalysts are currently under investigation, but they need to work together with a separate absorber for solar applications.15-20 MPc’s offer the possibility of combining the absorber and catalyst in a single molecule. Realizing the photocatalytic potential of MPcs requires careful tuning of their excited states to ensure that charge separation is sufficiently long to sustain a catalytic reaction. Recently, Gunaratne et al.21 have identified a relatively long-lived excited state in NiPc(OBu)8, the Ni(II) complex of Pc with eight butoxy substituents (Figure 1). They assigned the excited state as a LMCT state from its transient absorption spectral characteristics and the expected energy level ordering from TDDFT calculations. The reported ~600 ps lifetime of the LMCT state might permit ring reduction at a p-type semiconductor electrode, making Ni(I)Pc(OBu)8 available for substrate reduction. NiPc(OBu)8 has the added attraction of a substantially red-shifted visible absorption band (Figure 1), increasing its usefulness as a solar absorber. Structural characterization of the excited states of a potential catalyst is valuable in elucidating the complex mechanism of catalytic transformations. In contrast to transient absorption spectra that are broad and contain little structural information, vibrational spectra typically demonstrate narrow transitions whose frequencies are highly sensitive to structure. Access to ultrafast time-resolved vibrational spectra has recently been facilitated by the development of various coherent Raman spectroscopy (CRS) techniques, such as, coherent anti-Stokes Raman spectroscopy (CARS) and femtosecond stimulated 3 ACS Paragon Plus Environment
Journal of the American Chemical Society
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Raman spectroscopy (FSRS), which are capable of providing Raman spectra with both high spectral (
