Fluorescent coumarin derivatives with large stokes shift, dual emission and solid state luminescent properties: An experimental and theoretical study

Coumarin derivatives containing 8-benzothiazole (C-2) and its difluoroboron bound derivative (C-3) were prepared. Both derivatives show dual emission at 322 nm and 513 nm and large Stokes shift (188 nm), compared to the unsubstituted coumarin (C-1), which shows emission at 356 nm with small Stokes shift of 46 nm. C-2 and C-3 show fluorescence in solid state, in contrast the C-1 is non-fluorescent in the solid state. The excited state intramolecular proton transfer (ESIPT) process of C-2 was fully rationalized by DFT/TDDFT calculations with optimization of the ground state (S0) and excited state (S1) geometries. TDDFT calculations propose that the large Stokes shift of C-2 and C-3 are due to the re-distribution of the frontier molecular orbitals at excited states. Study of the potential energy curve of C-2 indicated that the dual emission of the C-2 is due to the simultaneous S1 and S3 emission, not the rotamer of the enol form.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Coumarin derivatives with large Stokes shift were prepared, based on ESIPT. ► The new derivatives are fluorescent at solid state. ► The ESIPT was rationalized by DFT calculations for the first time. ► Dual emission was observed for the new derivatives. ► DFT calculations propose that the dual emission is not due to the enol-rotamers.