Torsional disorder in tetraphenyl [3]-cumulenes: Insight into the excited state quenching.

Cumulenes are linear molecules consisting of consecutive double bonds linking chains of sp-hybridized carbon atoms. They have primarily been of interest for potential use as molecular wires or in other nanoscale electronic devices, but more recently other applications such as catalysis or even light harvesting through singlet fission have been speculated. Despite the recent theoretical and experimental interest, photoexcitation of cumulenes typically results in quenching on the picosecond timescale, and the exact quenching mechanism for even the simplest of [3]-cumulenes lacks a clear explanation. In this report, we perform transient absorption spectroscopy on a set of model [3]-cumulene derivatives in a wide range of environmental conditions to demonstrate that planarization of phenyl groups ultimately quenches the excited state. By restricting this intermolecular motion, we increase the excited state lifetime out to a few nanoseconds, strongly enhancing the photoluminescence and demonstrating an approach to stabilize them for photochemical applications.