Exceptional Excited-State Lifetime of an Iron(II)–N-Heterocyclic Carbene Complex Explained

Abstract

Earth-abundant transition-metal complexes are desirable for sensitizers in dye-sensitized solar cells or photocatalysts. Iron is an obvious choice, but the energy level structure of its typical polypyridyl complexes, featuring low-lying metal-centered states, has made such complexes useless as energy converters. Recently, we synthesized a novel iron-N-heterocyclic carbene complex exhibiting a remarkable 100-fold increase of the lifetime compared to previously known iron(II) complexes. Here, we rationalize the measured excited-state dynamics with DFT and TD-DFT calculations. The calculations show that the exceptionally long excited-state lifetime (∼9 ps) is achieved for this Fe complex through a significant destabilization of both triplet and quintet metal-centered scavenger states compared to other Fe(II) complexes. In addition, a shallow (3)MLCT potential energy surface with a low-energy transition path from the (3)MLCT to (3)MC and facile crossing from the (3)MC state to the ground state are identified as key features for the excited-state deactivation.

NHC-Based Iron Sensitizers for DSSCs

“…From the excited-state relaxation perspective, it is well known that the elongation of the Fe-ligand distance is a critical step in the spin cross-over process, and consequently, also for the MLCT/MC deactivation [27]. This is in line with the simple interpretation of the iron-based MC states as involving the population of the e g (i.e., antibonding orbitals).…”

“…Here, in addition to tuning of the ground-state properties (ligand field splitting), design strategies that influence the nonadiabatic excited-state relaxation kinetics [27] remain unexplored for the moment.…”

“…Recently, as shown by Wärnmark and coworkers, N-heterocyclic carbenes (NHC) have been proven to promote notable improvements of 3 MLCT lifetimes, thanks to strong σ-donating effects inducing the destabilization of MC states [25][26][27][28]. While no DSSCs have been built from the newly designed complexes, this work has fostered the revival of research towards the development of iron-based sensitizers for DSSCs, which has been almost asleep since the pioneering works by Ferrere using the Fe(dcbpy) 2 (CN) 2 dye [14].…”

From Mononuclear to Dinuclear: Magnetic Properties of Transition Metal Complexes

“…This is one of the reasons why more than twenty years after its discovery, LIESST e↵ect is still at the very center of the SCO research. The advances in short pulsed lasers enables researchers to extract information on the deactivation mechanism [15][16][17][18][19][20][21][22][23] and theoretical studies also contributed to the understanding of LIESST in octhedral Fe(II) complexes [60][61][62]. For Fe(III) complexes, the light irradiation induces ligand-tometal charge transfer transition (LMCT).…”

“…Therefore, the synthesis of systems with bistability under standard conditions for which switching is fast and both states are su ciently long-lived (i.e., persistent) has become increasingly important over the last 50 years. In this area of research, the interconversion between two di↵erent spin states that can be triggered by light irradiation is especially important [15][16][17][18][19][20][21][22][23]. The bistability in molecular magnets involves the M S components of a given spin state and the population of the bistable states can be controlled by the application of an external magnetic field.…”