Ultrafast Excited State Relaxation of a Metalloporphyrin Revealed by Femtosecond X-ray Absorption Spectroscopy

Abstract

Photoexcited Nickel(II) tetramesitylporphyrin (NiTMP), like many open-shell metalloporphyrins, relaxes rapidly through multiple electronic states following an initial porphyrin-based excitation, some involving metal centered electronic configuration changes that could be harnessed catalytically before excited state relaxation. While a NiTMP excited state present at 100 ps was previously identified by X-ray transient absorption (XTA) spectroscopy at a synchrotron source as a relaxed (d,d) state, the lowest energy excited state,1,2 structural dynamics before thermalization were not resolved due to the ~100 ps duration of the available X-ray probe pulse. Using the femtosecond (fs) X-ray pulses of the Linac Coherent Light Source (LCLS), the Ni center electronic configuration from the initial excited state to the relaxed (d,d) state has been obtained via ultrafast Ni K-edge XANES (X-ray Absorption Near Edge Structure) on a time scale from hundreds of femtoseconds to 100 ps. This enabled the identification of a short-lived Ni(I) species aided by time-dependent density functional theory (TDDFT) methods. Computed electronic and nuclear structure for critical excited electronic states in the relaxation pathway characterize the dependence of the complex’s geometry on the electron occupation of the 3d orbitals. Calculated XANES transitions for these excited states assign a short-lived transient signal to the spectroscopic signature of the Ni(I) species, resulting from intramolecular charge transfer on a timescale that has eluded previous synchrotron studies. These combined results enable us to examine the excited state structural dynamics of NiTMP prior to thermal relaxation and to capture intermediates of potential photocatalytic significance.

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

“…(a) XANES spectraofN iPcOBu8, along with the enlarged pre-edger egion in the inset. [19] In our studyo f NiPcOBu 8 ,ak inetic trace taken at 8.342 keV (Figure 7), which tracks the intensity of the new 1s!4p z transition originating from the photoexcited state, showedasingle exponential decay with at ime constant of 525 AE 27 ps. EXAFS spectra of grounds tate (black) and constructed excited state (blue) are shownin( b) k-space and (c) R-space.T he R-space data is not phase-corrected.…”

“…However,t he probed excited state of NiPcOBu 8 in this study revealed two pre-edge features arising at 8332 and 8334.5 eV, indicated by the difference spectrum as well as the extracted excited state spectrum.T his hints that the excited state probeda t1 00 ps after the 400 nm excitation is not ofaLMCT nature, but rather a( d,d) state in which both the 3d z 2 and 3d x 2 Ày 2 orbitals are singly occupied (Figure 6, right). [17][18][19] The blueshift of the 1s!4p z peak in the excited state also supported its assignment as a( d,d) state. [17][18][19] The blueshift of the 1s!4p z peak in the excited state also supported its assignment as a( d,d) state.…”

“…These two pre-edge features in the excited state spectrum of NiPcOBu 8 resemble that of the 3 (3d x 2 Ày 2 ,3 z 2 )s tate of NiTMP,w hich has been well-documented. The ultrafastX TA study on NiTMP [19] found that the short-lived LMCT state exhibited ar edshifted1 s !4p z peak compared to that of the ground state. The ultrafastX TA study on NiTMP [19] found that the short-lived LMCT state exhibited ar edshifted1 s !4p z peak compared to that of the ground state.…”

“…The ultrafastX TA study on NiTMP [19] found that the short-lived LMCT state exhibited ar edshifted1 s !4p z peak compared to that of the ground state. [17][18][19] The Ni 3d x 2 Ày 2 orbital has metal-ligand s antibonding character in the square-planar coordination geometry of tetrapyrrolic complexes. (a) XANES spectraofN iPcOBu8, along with the enlarged pre-edger egion in the inset.…”

“…The (p,d x 2 Ày 2 )s tate that exhibits Ni I characterw as successfully captured by the XTAs tudy using femtosecondX -ray pulseso ft he Linac Coherent Light Source, [19] whereas the relaxed (d z 2 ,d x 2 Ày 2 )s tate was evidenced by the 100 ps time-resolved XTAs tudies at Advanced Photon Source (APS). The (p,d x 2 Ày 2 )s tate that exhibits Ni I characterw as successfully captured by the XTAs tudy using femtosecondX -ray pulseso ft he Linac Coherent Light Source, [19] whereas the relaxed (d z 2 ,d x 2 Ày 2 )s tate was evidenced by the 100 ps time-resolved XTAs tudies at Advanced Photon Source (APS).…”