Lysosome-targeting solid state NIR emissive donor-acceptor molecules: a study on photophysical modulation through architectural distinction

The prevalence of the D-A strategy in achieving red-shifted emission has been established through designing D-A molecules of D-A-D and A-D-A constructs. Architectural control over such D-A systems integrates solid state NIR emission with lysosome tracking and sets a multifarious goal of photophysical modulation in a comprehensive way. In particular, two compounds, CPM-1 (D-A-D) and CPM-2 (A-D-A), have been synthesized by introducing carbazole-based donors and difluoroboron acceptors. Lysosome targeting and imaging have been achieved through incorporation of a morpholine unit, which ultimately imparts viscosity sensitivity to the construct. The fluorophores exhibited significant emission in solution along with distinctive solvatochromism, viscochromism and TICT. A comparative account of these competitive photophysical properties revealed the superior charge transfer properties of the A-D-A construct (CPM-2), while the D-A-D molecule (CPM-1) was found to be a better molecular rotor with marked viscochromism. The solid state NIR emission has been found to be much more intense in CPM-1 relative to CPM-2, which further highlights the influence of structural aspects on photophysical behvaiour. Theoretical studies further established the distinctive characteristics of ground and excited states in these compounds. Owing to its excellent viscochromic behvaiour, CPM-1 has been successfully utilized in lysosome targeting in wild-type Drosophila fly gut tissues through co-localization studies. The prevalence of the D-A strategy in achieving red-shifted emission has been established through designing D-A molecules of D-A-D and A-D-A constructs.