Alkali Blues: Blue‐Emissive Alkali Metal Pyrrolates

Abstract

2‐Iminopyrroles [HtBuL, 4‐tert‐butyl phenyl(pyrrol‐2‐ylmethylene)amine] are non‐fluorescent π systems. However, they display blue fluorescence after deprotonation with alkali metal bases in the solid state and in solution at room temperature. In the solid state, the alkali metal 2‐imino pyrrolates, M(tBuL), aggregate to dimers, [M(tBuL)(NCR)]2 (M=Li, R=CH3, CH(CH3)CNH2), or polymers, [M(tBuL)]n (M=Na, K). In solution (solv=CH3CN, DMSO, THF, and toluene), solvated, uncharged monomeric species M(tBuL)(solv)m with N,N′‐chelated alkali metal ions are present. Due to the electron‐rich pyrrolate and the electron‐poor arylimino moiety, the M(tBuL) chromophore possesses a low‐energy intraligand charge‐transfer (ILCT) excited state. The chelated alkali cations rigidify the chromophore, restricting intramolecular motions (RIM) by the chelation‐enhanced fluorescence (CHEF) effect in solution and, consequently, switch‐on a blue fluorescence emission.

Main Group Metal‐Mediated Transformations of Imines

“…Two general methods exist in the synthesis of 2‐imino pyrrolyl metal complexes, either direct in situ metallation with a strong base such as n BuLi, NaH or MHMDS (M=Li, Na or K; HMDS=hexamethyldisilazide) followed by transmetallation or an alkane elimination reaction. These methods have been exploited by the groups of Roesky, [43] Mashima, [44, 45] Panda [45–47] and Heinze [48] to access a range of alkali‐metal, Group 2, Al and Zn 2‐iminopyrrolate complexes.…”

“…Recently, Heinze [48] and co‐workers have reported a series of alkali metal 2‐iminopyrrolate complexes that show interesting fluorescent properties. Accessed through simple deprotonation with MHMDS (M=Li, Na or K), various aggregation states are observed in the solid state, while DOSY studies reveal all complexes exist as solvated monomers in solution (Scheme 10).…”

Intense Photoinduced Intervalence Charge Transfer in High‐Valent Iron Mixed Phenolate/Carbene Complexes

“…The current state-of-the-art to convert sunlight into electricity or fuels largely makes use of the 4d and 5d transition metals, which are typically more toxic, expensive, and scarce than their first-row congeners. Apart from the well-established use of the lanthanides, [1] this has resulted in the exploration of V, [2,3] Cr, [4][5][6] Mn, [7] Fe, [8][9][10][11][12] Co, [13] Cu [14][15][16] and Zr [17] as well as sblock complexes [18,19] for the application as emitters [20,21] or components in solar cells. [22][23][24] Amongst the 3d metals mentioned above, iron is particularly attractive due to its abundance, environmental friendliness and low toxicity.…”

Bright luminescent lithium and magnesium carbene complexes

“…The protonated proligand showed emissive properties as well (green, Φ em = 2.0%). Heinze et al 91 described, for example, “alkali-blue” emissive pyrrolates ( Fig. 1 , VI ).…”