Picosecond to Nanosecond Manipulation of Excited State Lifetimes in Complexes with an Fe^II to Ti^IV Metal-to-Metal Charge-Transfer: The Role of Ferrocene-Centered Excited States

Authors

Maksim Y. Livshits, University of New Mexico
Michael D. Turlington
Carl O. Trindle, University of Virginia
Lei Wang, University of New Mexico
Zikri Altun, Marmara University
Jeffrey J. Rack, University of New Mexico
Paul S. Wagenknecht, Furman UniversityFollow

ACS Citation

Livshits, M. Y.; Turlington, M. D.; Trindle, C. O. ; Wang, L. ; Altun, Z.; Wagenknecht, P. S.; Rack, J. J. Picosecond to Nanosecond Manipulation of Excited State Lifetimes in Complexes with an Fe^II to Ti^IV Metal-to-Metal Charge-Transfer: The Role of Ferrocene-Centered Excited States Inorg. Chem. 2019, 58, 15320 - 15329.

Version of Record

https://www.doi.org/10.1021/acs.inorgchem.9b02316

Abstract

Time-resolved transient absorption spectroscopy and computational analysis of D−π–A complexes comprising Fe^II donors and Ti^IV acceptors with the general formula ^RCp₂Ti(C₂Fc)₂ (where ^RCp = Cp*, Cp, and ^MeOOCCp) and ^TMSCp₂Ti(C₂Fc)(C₂R) (where R = Ph or CF₃) are reported. The transient absorption spectra are consistent with an Fe^III/Ti^III metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer (¹MMCT) excited state for the Fe^II/Ti^IV complexes is not purely MMCT in nature but is contaminated with the higher-energy ¹Fc (d-d) state. For the diferrocenyl complexes, ^RCp₂Ti(C₂Fc)₂, the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the ^MeOOCCp complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased ¹Fc character. The effect of Cu^I coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp₂Ti(C₂Fc)₂·CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of Ti^IV–³Fe^II–Cu^I.

Source Name

Inorganic Chemistry

Publication Date

11-5-2019

Volume

58

Issue

22

Page(s)

15320-15329

Document Type

Citation

Citation Type

Article