The Critical Role of nπ* States in the Photophysics and Thermally Activated Delayed Fluorescence of Spiro Acridine-Anthracenone

The molecular photophysics and thermally activated delayed fluorescence (TADF) in Spiro compounds are distinct because of the rigid orthogonal C-C bridging bond between donor and acceptor. The photophysics is found to be highly complex, with unprecedented multiple anti-Kasha emissions from three different singlet states, two of which are one-photon forbidden. The TADF mechanism is critically controlled by local acceptor n pi* states; the singlet n pi* state undergoes rapid intersystem crossing populating an energetically close acceptor pi pi* triplet state. The acceptor triplet n pi* state couples nonadiabatically to a CT triplet state mediating reverse intersystem crossing. When the n pi* and CT states are energetically close, TADF is greatly enhanced with rISC rate reaching 10(7) s(-1). We observe neither DF from the singlet n pi* state nor electron transfer (ET) to form the (CT)-C-1 because there is no ET driving force; however, ET from the higher-energy donor singlet pi pi* state readily occurs along with donor emission.

Automated summary

The molecular photophysics and TADF mechanism in Spiro compounds are unique due to the rigid orthogonal C-C bridging bond between donor and acceptor. The photophysics process is highly complex, with unprecedented multiple anti-Kasha emissions from different singlet states, while the TADF mechanism is critically controlled by local acceptor n pi* states.