Speaker
Description
Metalloporphyrins based on open-shell transition metals, such as Ni(II), exhibit typically fast excited-state relaxation. In this work, we shed light into the non radiative relaxation mechanism in a panchromatic nanographene-Ni(II) porphyrin conjugate. Variable temperature transient absorption spectroscopy (VTTAS) and global fit analysis are combined to produce a picture of the relaxation pathways.[1,2] The deciphered photophysical scenario comprises an intramolecular fast vibrational relaxation in 1.6 ps and subsequent intersystem crossing in 16 ps, bearing a short temporal window wherein singlets relax to the ground state, as confirmed by a short-lived NIR PL spanning from 850 to 1200 nm. Following intersystem crossing, fast relaxation of the triplet to the ground state proceeds in only 40 ps, almost an order of magnitude faster compared to Ni-based porphyrins. VTTAS combined with global analysis provide further information into this terminal relaxation stage. The results are interpreted in terms of a thermally induced population transfer from the lowest triplet energy level (a T(d,d) state according to DFT calculations) to a vibrationally excited ground state, enabled through a conical intersection of the respective ground and triplet potential energy surfaces. The findings presented herein showcase the remarkable potential of VTTAS in deciphering and quantifying rapid thermally activated relaxation phenomena.