Substituent effects on the aggregation-induced emission and two-photon absorption properties of triphenylamine-dibenzo[a, c] phenazine adducts

Exploration of high-performance fluorescent materials, especially those with two-photon absorption and aggregation-induced emission (AIE) properties, is of great significance to both fundamental research and practical applications. In the present work, a series of triphenylamine-dibenzo[a,c]phenazine adducts (Q1-Q5) with triphenylamine (TPA) moieties decorated by substituents ranging from nil to alkyl (methyl/octyl) and finally to alkoxy (methoxyl/octyloxy) groups were elaborately designed and facilely synthesized. Their photophysical properties including one- and two-photon absorption properties have been systematically investigated to clarify the relationships between their structures and properties and to see how a small change in the structure makes big differences in their performances. The proterotype triphenylamine-dibenzo[a,c]phenazine (TPA-DBP) adduct Q1 and the alkyl-substituted TPA-DBP adducts (Q2 and Q3) show intramolecular charge transfer (ICT) plus aggregation-enhanced emission (AEE) features while the alkoxy-decorated TPA-DBP adducts, i.e., Q4 and Q5, exhibit typical AIE behaviors. The differences in their photophysical properties can be mainly ascribed to the substituent effects, which are closely associated with the RIM (restriction of intramolecular motion) mechanism. Moreover, the AIE-active red luminogen Q5 with the largest two-photon absorption cross-section (σ = 801 GM) and high brightness has been further fabricated into nanoparticles via a simple and well-established method to satisfy the requirements of in vivo two-photon fluorescence imaging of blood vessels. The water-dispersible and biocompatible PEG-modified nanoparticles of Q5 performed well as an effective contrast agent for the visualization of blood vasculature with high signal-to-noise ratios, low photodamage and deep-tissue penetration capability (100 μm).