Phosphanylidenes for effective optical tuning of aromatic hydrocarbons

The development of novel concepts in photophysical tuning of organic molecules remains of significant importance. Here we demonstrate that simple carbene-stabilised phosphanylidene moieties, i.e. [NHC·P], drive large bathochromic shifts when bound to conjugated aromatic systems. Modulating the size of the conjugated scaffold from phenyl (2) to anthracenyl (5, 6) leads to the isolation of simple systems demonstrating absorption wavelengths of up to 650 nm. Simultaneously, increasing the number of [NHC·P] units in a given system significantly enhances molar absorptivities, reaching values of up to 16 000 L mol^-1 cm^-1. Computational analyses suggest that these photophysical characteristics arise from allowed π to π* transitions, that is P-centred lone pair donation into the aromatic π* orbital, i.e. [NHC·P] behaves as strong M+ donor, and effectively extends the conjugated π-system.