(Invited) Ultrafast Solar Energy Conversion with Earth-Abundant Complexes

Transition-metal complexes are widely used as photosensitizers and photocatalysts, and in light-emitting devices.[1] We have studied ultra-fast processes in a variety of DSSC and photocatalytic applications.[2,3] More recently we demonstrated the first iron complex [Fe III (btz) 3 ] 3+ exhibiting room temperature photoluminescence from a CT state and show that it has a 100 ps excited state CT lifetime which is unprecedented for any iron complex.[4] This was characterized as a rare low-spin Fe III d 5 complex with emission from a long-lived doublet ligand-to-metal charge transfer ( 2 LMCT) state. Absence of intersystem crossing (ISC) in this complex avoids significant excited state energy losses as encountered in the prevailing class of d 6 TM complexes of e.g. Fe II and Ru II . Furthermore, we have also shown [Fe II (btz) 3 ] 2+ low-spin complex [ 5 ], a Fe II analogue to [Fe III (btz) 3 ] 3+ . It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3 MLCT state with a 528 ps excited state lifetime in CH 3 CN solution that is more than one order of magnitude longer compared to the MLCT lifetime of any previously reported Fe II complex. Together, these results show that the Fe II and Fe III oxidation states of the same Fe(btz) 3 complex feature long lived MLCT and LMCT states, respectively. This demonstrates the versatility of iron N- heterocyclic carbene complexes as promising light-harvesters for a broad range of photophysical and photochemical applications, and on-going efforts for further improvements of the excited state dynamics of such systems will be outlined, including the very latest development of an iron compound [Fe(phtmeimb) 2 ] + with an excited state stable enough to emit luminescence with unprecedented 2 ns lifetime and with the ability to engage in bi-molecular electron transfer [6]. References: 1 Ponseca, C. S., Jr.; Chabera, P.; Uhlig, J.; Persson, P.; Sundstrom, V. Ultrafast Electron Dynamics in Solar Energy Conversion. Chem Rev 2017, 117 (16), 10940. 2 H. Imahori et al, Photoinduced Charge Carrier Dynamics of Zn−Porphyrin−TiO2 Electrodes: The Key Role of Charge Recombination for Solar Cell Performance, J. Phys. Chem. A 2011, 115 , 16, 3679-3690 3 M. Yamamoto et al, Visible light-driven water oxidation with a subporphyrin sensitizer and a water oxidation catalyst, Chem. Commun. 2016, 52 , 13702-13705 4 Chabera, P. et al. A low-spin Fe(III) complex with 100-ps ligand-to-metal charge transfer photoluminescence. Nature 2017, 543 (7647), 695. 5 Chabera, P. et al. Fe(II) Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime, J Phys Chem Lett 2018, 9 , 459−463. 6 Kjær, K. S. et al. Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science 2019, 363 (6424), 249-253. Figure caption: Photoluminescent iron complex [6] Figure 1