The Superiority of Nonconjugated Structures in Fluorescence: Through-Space vs. Through-Bond Charge Transfer

Constructing charge transfer (CT) state by introducing donor (D) and acceptor (A) is an efficient strategy to regulate the photophysical properties of luminescent materials. Traditional CT-type luminophores are built on π-conjugated fused-ring structures, which always show hybrid CT/locally excited (LE) states and luminescence quenching effect in the aggregate state. In this work, eight conjugated biphenyl (BP) and nonconjugated diphenylmethane (DPM) derivatives with different donors and acceptors are synthesized to investigate the CT properties. Systematic photophysical characterization and theoretical calculation demonstrate that the through-space CT (TSCT) in nonconjugated DA-DPM exhibit superior photophysical performance than the conjugated DA-BP with through-bond CT (TBCT), the main manifestations are as follows: i) TSCT luminophores produce longer maximum emission wavelength (λem) than the corresponding TBCT ones, for example, the longest λem of DMA-CN-DPM is 621 nm but the corresponding λem of DMA-CN-BP is only 480 nm. ii) TSCT-based DA-DPM demonstrates more sensitive responsiveness to environmental stimuli such as temperature and polarity. iii) Complete charge separation exists in all kinds of conformation of DA-DPM, which was hard to realize in conjugated DA-BP.