Through-Space Charge Transfer Dynamic Mechanism in V-Shaped Flexible Carbazole Aromatic Imides Dyads

Cofacial electron donor-acceptor dyads exhibiting through-space charge-transfer (TSCT) characteristics are widely employed in the development of optoelectronic functional materials. The flexible molecular frameworks between the electron donor and acceptor components allow dynamic modulation of electronic coupling, influenced by excited-state structural relaxation or intermolecular interactions, thereby affecting the charge-transfer (CT) dynamics and the emission properties of TSCT states. In this work, we examine the TSCT dynamic processes of two electron donor-acceptor dyads, CzPhNI and CzPhPI formed by ortho-substitution of phenyl linkage with V-shaped flexible TSCT structures using carbazole as donor and naphthalimide or phthalimide as acceptor. A pseudo-cofacial TSCT conformation formed in the excited state effectively shortens the donor-acceptor distance and enhances CT coupling. Femtosecond spectroscopy reveals an ultrafast TSCT kinetics in the (200 fs)^-1 timescale. Moreover, the intermolecular interaction-induced D-A stacking further strengthens the electronic coupling, accelerating the TSCT reaction rate from (2.07 ps)^-1 to (174 fs)^-1 as solution concentrations increases from 10^-5 to 10^-3 M. The results obtained in this work offers valuable physical insights into the TSCT dynamic mechanism, potentially explaining the widely observed emission enhancement in weakly-coupled, flexible TSCT emitters transitioning from solution to aggregation states.