Vibrationally Hot (dd) Excited States of Ni^II-Porphyrins Probed by Picosecond Time-Resolved Resonance-Raman Spectroscopy

Abstract

Ni^II-porphyrins have served as an exemplary system for elucidating dynamical features involved in photoexcitation and relaxation of metalloporphyrins [1–5]. The recent subpicosecond time-resolved absorption study [4] of Ni^II-porphyrins in non-coordinating solvents has demonstrated that the $\left( {{{d}_{{{{z}^{2}}}}},{{d}_{{{{x}^{2}} - {{y}^{2}}}}}} \right)$ states are generated within 350fs of the (ππ*) excitation of the macrocycle and decays with biphasic kinetics as reported (τ₁ = ~10ps and τ₂= 250−300ps) [1,3]. While the τ₂ relaxation is established to be an electronic decay process of the (dd) excited state, the τ₁ relaxation is a concern of current photodynamic investigations. Earlier studies proposed this to be an electronic relaxation from (ππ*) to (dd) [1], or from ¹(dd) to ³(dd) [3], but recent studies assigned it to non-electronic relaxations within the (dd) manifolds [4–6], since the value of τ₁ is varied with monitoring wavelengths of the transient absorption. The value of τ₁ (~5ps) at the red-edge of the transient absorption is almost the half of the value (~10ps) measured at the blueedge[4]. Rodriguez et al. proposed it to be a vibrational cooling process [4,5] of the vibrationally hot (dd)-excited states generated when the potential of (ππ^*) crosses that of (dd), while Courtney et al. proposed it to be an conformational relaxation on the basis of time-resolved resonance Raman (TR³) spectra measured in the τ₂ region [6]. Ni^II-porphyrins are known to have planar- and ruffled-conformers in solution, and several Raman lines of the in-plane skeletal vibrations of porphyrin macrocycle (ν₁₀, ν₂ and ν₃) are apparently broadened (г = 16–25cm⁻¹) in the ground electronic state [7], although Courtney et al. observed that these modes become sharp (10–14cm⁻¹) in the TR³ spectra of the τ₂ region [6]. This fact suggests that Ni^II-porphyrins will have only a single conformer in the (dd) state in the τ₂ region, and so as to make it possible, there should be some conformational relaxation in the earlier time range. Courtney et al. attributed the τ₁ process to it [6]. However, the problem seems to be still under debates [5]. We report here the measurement of TR³ spectrum in the τ₁ region which is indicative of the vibrational cooling for the τ₁ process, although the possibility that the conformational relaxation coexists cannot be ruled out completely.