Aggregation-Induced Delayed Fluorescence and Phosphorescence from Hot Excitons via Suppression of Kasha’s Rule in a Stimuli-Active Molecular Rotor

This
work presents a series of through-space donor-alkyl bridge-acceptor
(D-σ-A) luminogens, BT2OxCz [2,2′-(5-(3-(9H-carbazol-9-yl)alkoxy)-1,3-phenylene)bis(benzothiazole)]
(x = 3, 4, 5, and 6), as anti-Kasha emissive multifunctional
molecular rotors with in-depth mechanistic insights, conveyed by experimental
and theoretical results. The origin of aggregation-induced emission
and coexistence of dual fluorescence and phosphorescence are realized
from hot excitons (especially from the bright emissive states of S7/S5 and T3) by suppressing Kasha’s
rule. More importantly, the large energy gaps (ΔEST) between S7/S5 and S1 states (T3 and T1 states) along with an abundance
of small ΔEST between singlet charge
transfer and triplet locally excited states facilitate reverse intersystem
crossing resulting in thermally activated delayed fluorescence and
room-temperature phosphorescence simultaneously. Besides, alkyl chain
length-induced distinct H- and J-aggregation offers tunable triplet-harvesting
behavior and white-light emission along with two different reversible
mechanoresponsive properties viz. mechanoluminescence and mechanochromism,
respectively.