The Overlooked Dual Phosphorescence of Lappert’s Diamino Stannylene Sn[N(SiMe3)2]2

The first stable heavy carbene homologues, the heavy tetrylenes, were reported in 1973 by Lappert and co‐workers. These tetrylenes were extensively investigated with respect to ground state reactivity such as small molecule activation, insertion into σ‐bonds, coordination chemistry, materials chemistry or catalysis. Their photophysical properties remained essentially unexplored. We report that Lappert’s bright yellow coloured diamino stannylene Sn[N(SiMe3)2]2 shows thermally activated dual orange/green phosphorescence with microsecond lifetime in fluid solution at room temperature, which has been overlooked for more than 50 years. These unique electronic and photophysical properties are studied in detail by temperature‐dependent time‐resolved emission and absorption spectroscopy and are corroborated by (time‐dependent) density functional theory calculations. The mechanism of photochemical radical formation has been disclosed involving unprecedented stannylene excimers with second‐order Jahn‐Teller distorted structures. The present study provides new insights towards a rational design of tetrel(II) complexes with long‐lived emissive excited states with Lappert’s diamino stannylene being the prototype.