Pyrene-Decoration of a Chromium(0) Tris(diisocyanide) Enhances Excited State Delocalization: A Strategy to Improve the Photoluminescence of 3d⁶ Metal Complexes

Abstract

There is a long-standing interest in iron­(II) complexes that emit from metal-to-ligand charge transfer (MLCT) excited states, analogous to ruthenium­(II) polypyridines. The 3d6 electrons of iron­(II) are exposed to a relatively weak ligand field, rendering nonradiative relaxation of MLCT states via metal-centered excited states undesirably efficient. For isoelectronic chromium(0), chelating diisocyanide ligands recently provided access to very weak MLCT emission in solution at room temperature. Here, we present a concept that boosts the luminescence quantum yield of a chromium(0) isocyanide complex by nearly 2 orders of magnitude, accompanied by a significant increase of the MLCT lifetime. Pyrene units in the diisocyanide ligand backbone lead to an enlarged π-conjugation system and to a strongly delocalized MLCT state, from which nonradiative relaxation is less dominant despite a sizable redshift of the emission. While the pyrene moiety is electronically coupled to the core of the chromium(0) complex in the excited state, UV–vis absorption and 2D NMR spectroscopy show that this is not the case in the ground state. Luminescence lifetimes and quantum yields for our pyrenyl-decorated chromium(0) complex exhibit an unusual bell-shaped dependence on solvent polarity, indicative of two counteracting effects governing the MLCT deactivation. These two effects are identified as predominant deactivation either through an energetically nearby lying metal-centered state in the most apolar solvents, or alternatively via direct nonradiative relaxation to the ground state following the energy gap law in more polar solvents. This is the first example of a 3d6 MLCT emitter to benefit from an increased π-conjugation network.

Luminescent chromium(0) and manganese(i) complexes

“…1 ) has substantially improved photophysical properties relative to the [Cr(L H ) 3 ] parent complex without anchored pyrene moieties ( Table 1 ). 20 Its (average) 3 MLCT lifetime of 6.10 ns in deaerated cyclooctane at 20 °C is, to the best of our knowledge, the longest 3 MLCT lifetime reported to date for any 3d 6 complex under comparable conditions. Recent record 3 MLCT lifetimes (of dark, non-emissive states) for isoelectronic Fe II complexes are 528 ps and 2.6 ns.…”

“…The MLCT luminescence lifetimes and quantum yields for [Cr(L pyr ) 3 ] exhibit an unusual bell-shaped dependence on solvent polarity indicative of two counteracting effects governing the MLCT deactivation, 20 a behavior that is not typically seen for precious metal-based 4d 6 and 5d 6 MLCT luminophores. Polar solvents stabilize the emissive 3 MLCT state of [Cr(L pyr ) 3 ] and thereby decrease the energy gap to the electronic ground state, which accelerates non-radiative relaxation.…”

Ultrafast Spectroscopy of Fe(II) Complexes Designed for Solar‐Energy Conversion: Current Status and Open Questions

“…[116] Even though each metal is different, one should not forget to look across the borders and find inspiration in the photo-chemistry of other first row transition metals. [3][4][5][6][7][8][9][10][11][12][13]…”

“…In the past ten years, research on coordination complexes based on first row transition metals has witnessed an increasing attention, driven by the ambitious aim of conferring these materials with exploitable photophysical properties such as high fluorescence/luminescence quantum yields or long enough excited state lifetimes (ESL) useful for photocatalysis or other photochemical applications. The main challenge is to replace scarce, and thus expensive, transition metal complexes with high ligand field splitting energies (LFSE), such as Ru, Ir or Pt, by abundant and cheaper metals, such as Fe, Cu, Cr, Zr or Mn [1–13] …”

Bidentate Pyridyl‐NHC Ligands: Synthesis, Ground and Excited State Properties of Their Iron(II) Complexes and the Role of the fac/mer Isomerism

“…However, it is worth to note that isoelectronic 3d 6 MLCT emitters can indeed be obtained by replacing the central metal as recently reviewed. [39,40] For instance, this original approach has been successfully demonstrated very recently for Cr 0 [41,42] or Mn I . [43] Two examples are shown in Figure 1 (complexes 6 [41] and 7 [43] ).…”