Effect of π-conjugation on solid-state fluorescence in highly planar dyes bearing an intramolecular H-bond

Efficient solid emitters with planar architectures are desirable for some practical applications. However, molecules with coplanar structures and conformational rigidity generally exhibit small Stokes shifts, thus resulting in self-absorption. Multi-aromatic hydrocarbons are one typical example that easily undergoes aggregation-caused quenching (ACQ) due to intermolecular pi-pi interactions. Herein, a highly planar seven-ringed dye containing benzothiazole-and triphenyl-fused moieties (HTpT) without any bulky or propeller-shaped substituent at its periphery was facilely synthesized from a Cu+-catalyzed Scholl reaction. By extending the phenyl in excited-state intramolecular proton transfer (ESIPT)-active dye (HBT) to naphthyl (HNT) and to triphenyl (HTpT), HNT shows hypsochromic emission and small Stokes shift (20 nm) in comparison with parent HBT, and suffers from ACQ. Contrarily, HTpT exhibits bright red solid-state fluorescence (lambda(em): 600 nm, Phi(f): 22.7%) with a 200 nm Stokes shift. Steady-state fluorescence spectra, theoretical calculations, and X-ray diffraction analysis are performed to elucidate these different photo-physical properties. In addition, unlike most lysosomal targeting probes containing proton-binding amino or morpholine moieties, which can cause an alkalinizing effect that interferes with the activity of normal cells, HTpT without a basic amino group can specifically light up cellular lysosomes.