The Nature of the Long‐Lived Excited State in a Ni^II Phthalocyanine Complex Investigated by X‐Ray Transient Absorption Spectroscopy

Abstract

The nature of the photoexcited state of octabutoxy nickel(II) phthalocyanine (NiPcOBu ) with a 500 ps lifetime was investigated by X-ray transient absorption (XTA) spectroscopy. Previous optical, vibrational, and computational studies have suggested that this photoexcited state has a ligand-to-metal charge transfer (LMCT) nature. By using XTA, which provides unambiguous information on the local electronic and nuclear configuration around the Ni center, the nature of the excited state of NiPcOBu was reassessed. Using X-ray probe pulses from a synchrotron source, the ground- and excited-state X-ray absorption spectra of NiPcOBu were measured. Based on the results, we identified that the excited state exhibits spectral features that are characteristic of a Ni (3dz2 ,3dx2-y2 ) state rather than a LMCT state with a transiently reduced Ni center. This state resembles the (d,d) state of nickel(II) tetramesitylphorphyrin. The XTA features are rationalized based on the inherent cavity sizes of the macrocycles. These results may provide useful guidance for the design of photocatalysts in the future.

Sub-100 fs Intersystem Crossing to a Metal-Centered Triplet in Ni(II)OEP Observed with M-Edge XANES

“…10−12 In noncoordinating solvents, such as toluene, photoexcited Ni porphyrins relax in hundreds of ps. [7][8][9]13,14 In the presence of coordinating solvents, such as pyridine and piperidine, Ni porphyrins undergo photoinduced axial ligation several hundred ps after excitation to populate a six-coordinate 3 (d,d) excited-state species with lifetimes of tens of ns. 7,12 Work on four-and six-coordinate nickel porphyrins, primarily via ultrafast transient optical spectroscopy and K-edge XAS, has characterized the effects of axial ligation on the ground and excited electronic structures.…”

Novel Integration of Nickel Phthalocyanine/Nickel Oxide-Based Photocathodes and Copper-Encapsulated Carbon-Dot-Cosensitized Photoanodes in Tandem for a Highly Efficient Solar Cell

“…Porphyrin derivatives particularly phthalocyanines in combination with the metal oxide semiconductors have been successfully employed in fabricating most efficient p-or n-type DSSCs to date 15−17 by exploiting lower-energy photons of the near-red region and transferring energy or electrons between the macrocycle and the bound metal. 18 By virtue of being cost-effective, ecofriendly, and sensitive to longer wavelengths of the electromagnetic spectrum, electron-accepting metallophthalocyanines have long been considered as a class of attractive light-harvesting nanomaterials. 19 Herein, nickel phthalocyanine-tetrasulfonic acid tetrasodium salt (NiPcTs) has been investigated as a potential sensitizer and a prospective alternative to inorganic light harvesters because of its excellent spectral behavior in the red and near-infrared range.…”

“…The valence-band (VB) edge of NiO has made it one of the most appropriate electron donors for many photosensitizers as testified in several reports over time. − The advantage of this VB positioning lies in the increase in the open-circuit voltage ( V OC ) of the tandem device, which relies on the difference between the quasi-Fermi levels of the NiO- and TiO 2 -based electrodes. Porphyrin derivatives particularly phthalocyanines in combination with the metal oxide semiconductors have been successfully employed in fabricating most efficient p- or n-type DSSCs to date − by exploiting lower-energy photons of the near-red region and transferring energy or electrons between the macrocycle and the bound metal . By virtue of being cost-effective, ecofriendly, and sensitive to longer wavelengths of the electromagnetic spectrum, electron-accepting metallophthalocyanines have long been considered as a class of attractive light-harvesting nanomaterials .…”

Sub-100 Fs Intersystem Crossing to a Metal-Centered Triplet in Ni(II)OEP Observed with M-Edge XANES

“…6 Previous studies have shown that the unique radiationless relaxation mechanism of nickel(II) porphyrinate species is due to the involvement of a long-lived d-d excited state, possibly through a short-lived LMCT intermediate. [7][8][9] The functionality of metalloporphyrins relies heavily on their ability to form transient or stable axial bonds and full understanding of their function necessitates characterization of the ground and excited metal electronic structure at every step in the relaxation process. Photoexcitation of the Soret or Q-band of Ni porphyins populates the singlet S 1 /S 2 state followed by energy transfer into the metal d-manifold, forming a metal-centered (d,d) excited state that can either relax directly to the ground electronic state or form a transient but relatively long-lived spin-triplet sixcoordinate species.…”

“…[10][11][12] In noncoordinating solvents such as toluene, photoexcited Ni porphyrins relax in hundreds of ps. [7][8][9]13,14 In the presence of coordinating solvents such as pyridine and piperidine, Ni porphyrins undergo photoinduced axial ligation several hundred ps after excitation to populate a six-coordinate 3 (d-d) excited state species with lifetimes of tens of ns. 7,12 Work on four-and six-coordinate nickel porphyrins, primarily via ultrafast transient optical and K-edge XAS, has characterized the effects of axial ligation on the ground and excited electronic structure.…”