Molecular Engineering to Tune Functionality: The Case of Cl-Substituted [Fe(terpy)₂]²⁺

Abstract

The properties of transition-metal complexes and their chemical dynamics can be effectively modified with ligand substitutions, and theory can be a great aid to such molecular engineering. In this paper, we first theoretically explored how substitution with a Cl atom at different positions of the terpyridine ligand affects the electronic structure of the [Fe(terpy) 2 ] 2+ complex. We found that besides the substitution at position 4′, the next most promising candidate to cause substantial electronic effects is that where the side pyridine ring is substituted at position 5 (β). Therefore, next, we examined in detail the Fe(II) complexes of the 5-chloro and 5,5″-dichloro derivatives of terpy, theoretically and experimentally, to reveal how these substitutions modify the ground state properties and the lifetime of the excited quintet state in such complexes. In addition, we extend the investigation to the complexes of the analogously substituted derivatives of 4′-SMeterpy. The substitution at position(s) 5 (and 5″) with Cl lowers the energy of the quintet state and increases its lifetime; the results on the 4′-SMe-substituted complexes show similar changes with these two substitutions, verifying that these effects are more or less additive. This study contributes to the enhancement of our molecular engineering toolset for modifying the potential energy landscape of similar complexes.

Ultrafast 3MLCT quenching and vibrational coherence: excited-state dynamics of the first-discovered Fe(ii)–carbene sensitiser resolved

“…Ideally, a balanced multiconfigurational treatment such as multiconfigurational second-order perturbation theory (CASPT2) would be the method of choice, however, computational feasibility issues as well as variance of the utilised active space for the nuclear configurational space explored by the dynamics severely limit its applicability. Herein, we use our efficient DFT/TD-DFT framework based on the B3LYP* exchange-correlation functional, 23 which we benchmarked extensively against experimental 24,25 and CASPT2 data. 12,26 We carry out TSH dynamics simulations both on DFT/TD-DFT PESs, according to a linear vibronic coupling (LVC) harmonic model 27 (which allows simulations up to several ps), and full on-the-fly ab initio PESs (in order to assess the validity of the LVC model for short sub-ps timescales).…”

Determining the Quintet Lifetimes in Side-ring Substituted [Fe(terpy)2]2+ Complexes

“…The subject of the present study is the effect of symmetric substitution in the 5,5" positions of the side-rings. The Cl substituent was previously shown to act in a similar manner in both the 4', and the 5,5" positions, lengthening the quintet lifetime in both cases [20]. However, as Cl is a substituent with a medium-strength −I and a weak +M effect, we have concluded that groups with stronger effects need to be studied as well, since we may expect interesting behavior from these.…”

“…We synthesized the complexes and followed their photorelaxation using transient optical absorption spectroscopy (TOAS). Based on the calculations and previous experimental results [15,18,20], it can be safely assumed that the final relaxation step with lifetimes in the nanosecond range belongs to the quintet-singlet relaxation process in all complexes discussed in this paper. We also provide calculation-based predictions of these lifetimes using simple, but qualitatively accurate methods to further verify their applicability in case of side-ring substitutions.…”

“…We previously studied the effect of electron-withdrawing ( EW ) and donating ( ED) substituents on the quintet-state lifetime of the [ Fe(4'-R-terpy) 2 ] 2+ system [19] and conducted a rigorous study on the effects of the Cl-substituent at various positions [20]. These studies have confirmed that modifications at positions with para-relation to the axial nitrogen ( N ax ) in the central ring can induce significant changes in the potential-energy surface (PES).…”

“…We also provide calculation-based predictions of these lifetimes using simple, but qualitatively accurate methods to further verify their applicability in case of side-ring substitutions. UV-Visible spectroscopy measurements were also performed as the positions of the absorption bands pertaining to the MLCT transitions had previously also been shown to be dependent on the EW/ED effects of the substituent moieties [20].…”