Ultrafast branching in intersystem crossing dynamics revealed by coherent vibrational wavepacket motions in a bimetallic Pt($$\mathrm{II}$$) complex

Journal Article · Wed Jun 01 00:00:00 EDT 2022 · Faraday Discussions

DOI:https://doi.org/10.1039/d2fd00009a· OSTI ID:1923256

Kim, Pyosang  ^[1]; Valentine, Andrew S. ^[2]; Roy, Subhangi ^[3]; Mills, Alexis W ^[2]; Castellano, Felix N.  ^[3]; Li, Xiaosong  ^[2]; Chen, Lin X.  ^[1]

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1. Northwestern University, Evanston, IL (United States); Argonne National Lab. (ANL), Lemont, IL (United States)
2. University of Washington, Seattle, WA (United States)
3. North Carolina State University, Raleigh, NC (United States)

Ultrafast excited-state processes of transition metal complexes (TMCs) are governed by complicated interplays between electronic and nuclear dynamics, which demand a detailed understanding to achieve optimal functionalities of photoactive TMC-based materials for many applications. Here, in this work, we investigated a cyclometalated platinum(II) dimer known to undergo a Pt-Pt bond contraction in metal-metal-to-ligand-charge-transfer (MMLCT) excited state using femtosecond broadband transient absorption (fs-BBTA) spectroscopy in combination with geometry optimization and normal mode calculations. Using sub-20 fs pump and broadband probe pulses in fs-BBTA spectroscopy, we are able to correlate coherent vibrational wavepacket (CVWP) evolution with stimulated emission (SE) dynamics of the ¹MMLCT state. The results demonstrated that the 145 cm^-1 CVWP motions with the damping times of ~ 0.9 ps and ~ 2 ps originate from coherent Pt-Pt stretching vibrations in the singlet and triplet MMLCT states, respectively. On the basis of excited-state potential energy surface calculations in our previous work, we rationalized that the CVWP transfer from Franck-Condon (FC) state to the ³MMLCT state was mediated by a triplet ligand-centered (³LC) intermediate state through two step intersystem crossing (ISC) on a time scale shorter than a period of the Pt-Pt stretching wavepacket motions. Moreover, it was found that the CVWP motion with 110 cm-1 frequency decays with the damping time of ~ 0.2 ps, well matching the time constant of 0.253 ps corresponding to a redshift in the SE feature at early time. This observation indicates that the Pt-Pt bond contraction changes the stretching frequency from 110 to 145 cm^-1 and stabilize the ¹MMLCT state relative to the ³LC state with the ~ 0.2 ps time scale. Thus, the ultrafast ISC from the ¹MMLCT to the ³LC states occurs before the Pt-Pt bond shortening. The findings herein provide insight into understanding the impact of Pt-Pt bond contraction on ultrafast branching of the ¹MMLCT population into the direct (¹MMLCT → ³MMLCT) and indirect ISC pathways (¹MMLCT → ³LC → ³MMLCT) in the Pt(II) dimer. These results revealed intricate excited state electronic and nuclear motions that could steer the reaction pathways with levels of details that have not achieved before

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division; National Science Foundation (NSF); USDOE
• Grant/Contract Number: AC02-06CH11357; CHE-1955806; CHE-1955795
• OSTI ID: 1923256
• Alternate ID(s): OSTI ID: 1870732
• Journal Information: Faraday Discussions, Vol. 237; ISSN 1359-6640
• Publisher: Royal Society of Chemistry
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY