Visualizing\nThermally Activated Conical Intersections\nGoverning Non-Radiative Triplet Decay in a Ni(II) Porphyrin-Nanographene\nConjugate with Variable Temperature Transient Absorption Spectroscopy

Metalloporphyrins based on open-shell transition metals,\nsuch as\nNi(II), exhibit typically fast excited-state relaxation. In this work,\nwe shed light into the nonradiative relaxation mechanism in a nanographene-Ni(II)\nporphyrin conjugate. Variable temperature transient absorption and\nglobal fit analysis are combined to produce a picture of the relaxation\npathways. At room temperature, photoexcitation of the lowest π–π*\ntransition is followed by vibrational cooling in 1.6 ps, setting a\nshort 20 ps temporal window wherein a small fraction of relaxed singlets\nradiatively decay to the ground state before intersystem crossing\nproceeds. Following intersystem crossing, triplets relax rapidly to\nthe ground state (S<sub>0</sub>) in a few tens of picoseconds. By\nperforming measurements at low temperature, we provide evidence for\na competition between two terminal relaxation pathways from the lowest\n(metal-centered) triplet to the ground state: a slow ground state\nrelaxation process proceeding in time scales beyond 1.6 ns and a faster\npathway dictated by a sloped conical intersection, which is thermally\naccessible at room temperature from the triplet state. The overall\ntriplet decay at a given temperature is dictated by the interplay\nof these two contributions. This observation bears significance in\nunderstanding the underlying fast relaxation processes in Ni-based\nmolecules and related transition metal complexes, opening avenues\nfor potential applications for energy harvesting and optoelectronics.